Summit, I suppose I should really let mkhoot be the one to provide the most commentary since it is his wonderful model and I'd like to hear some more myself.
What I can tell you is that your task may be easier than you think. I believe he used an AMT Ertl 68 Roadrunner here as his base kit and modified and added to it. I see he opened up the inner fenders but retained their basic structure for strength and added bracing and mounts for the shocks. It also looks like he found some Holman Moody wheels, and some MPC 1970 and up Goodyear tires which are slightly different than the 68s but are a nicely detailed, better looking and more aggressive sort of tire if 100% accuracy doesn't bother you.
Now, going to my own experience and advice, so as to not further step on any toes or make guesses on mkhoots great car,the wheels to which I refer were in many early MPC stock car kits and the tires were in a plethora of kits, including all the Model King MPC reissues of that series out now (Torino, Cyclone, Laguna, Monte Carlo) and numerous odd balls have pairs of them, like the current AMT 72 GTO (which also contains an extreemly detailed replica of a NHRA pro starting line Christmas tree) and the mislabeled 75 Duster (really a Dart Sport.)
If you can pick up a swap meet 70's MPC Annual Camaro kit, they all had these wheels for their stock car option which I think the kits had every year from at least 1973 through 81 and the bonus there is the roll cage. It is pretty accurate, you can modify it a bit by adding some bars unique to Richards Cars but the "score" you make here is it fits on the interior floor and inside of those AMT ERTL Roadrunner and GTX kits like it was made for them, right height,width and all, no trimming necessary. I'm using this on my black topped Petty Roadrunner discussed elsewhere in this thread and have built a short track version using these parts already. The Camaro also contains headers that fit the Hemi 9though there are good headers included on the tree in the Roadrunner kit) and the bathtub or open plenum type intake manifold you need to top off the Hemi, for the single 4 bbl. is also in the Camaro.
The interior in the Roadrunner is great because you can leave the inner door panels off, or, turn them around and sand and fill them flat and slice off and apply only the window cranks to be perfectly accurate. There are no seats molded in saving you a ton of work. Its perfect for a stock car. Further the chassis is accurate since there is no exhaust detail molded in. You can turn the front king pins upside down to lower the front suspension slightly and thats all you should have to do there. Add some aluminum tube exhaust pipes and your are good to go. Bottom line, if you just wanted to scratch build a roll cage and order some HM wheels from the Modelhaus or Plastic Performance Products,(the Decal Man and BSR carry these on their sites as well as a generic cage which is nice but has to be narrowed quite a bit to fit in anything but a Ford Galaxie) order a set of decals from one of numerous on line sources and paint with Tamiya Gloss Light blue or Testors model master laquor system AMC Big Bad Blue you would be all set, with a model more accurate and with less work in it and for a far lower price than a 68 or 69 Johan if you can find one. The suspensions are far more detailed than the molded in ones on the Johans, you'd also have to do a ton of work to remove the molded in stock exhaust from the Johans, the wheels aren't entirely accurate with those (though they are real good representations of early Mopar Super stock drag racing steel wheels) and the headers and engines would be more up to date than in the Johans. The Johans of any year however, do have the absolutely accurate tires for the 68 Roadrunner.
On the other hand, if you want a Petty Superbird, the ebay or other auction route on the net is the only way to go, they are your only option and are rather plentiful as of late. So are the 64 Belvederes, (I've not yet seen any of the Lindberg versions yet, but I'm sure they may be seen at any time. The Johan kits will require the extra work as I described, but for those years they're the only game in town and you do of course get exquisite engine detail, crisp beautifully proportioned bodies, great racing dashboards, crude but modifiable roll cages and sturdy easy construction. These years are more plentiful and not as pricy.
For a 67 or 66 Petty car, use the revell GTX kit. You'll need to gather many of the parts I described above, but except for the intake manifold, needing a roll cage and a different stripped down interior, (the 68-69 AMT units modified a bit might work as well as the one, cut and modified abit in the Allison AMT Matador Stock cars that you still see around for a song) this is a very doable project for anyone beyond an absolute beginner.The cage in the Matador can be made to work too, an added bonus. A little, very little, opening of the wheel openings may be needed to fit the Holman Moodys and bigger rubber.
You'd have to turn to the aftermarket, like the Modelhaus or Chuck Poi's Race Ready Replicas for earlier cars mainly. Johan 63 , 62, 61, 60 and 59 Furys can be found on the net but for generally exhorbitant prices and would require lots of conversion work, though Chuck has all you need for a 63 practically, including wheels, tires, interior,dash, seats and complete body with glass bumpers grilles and all. From 71 on you've got aftermarket sources for the 71 and 72 Plymouth bodies all of which would mount up on those MPC Nascar/Southern Stocker type adjustable chassis which, while compromised in certain ways to be generic for many bodies, are very detailed, don't be fooled. The Charger bodies and the Magnum (which you'd have to find in the aftermarket), even a 77-78 Revell Monte Carlo low rider body could go on these. All the Chargers were kitted as Petty cars with this Chassis by MPC and can be found on net. Polar lights had a strange but in the end maybe not so bad Charger too. Starting in 81, Monogram, Revell took care of every body style through the Kings retirement in detailed kit form except for the modified Grandprix fastback 2+2, but body conversions abound in the after market to put on the slope nose, fastback glass and rear clip which are cheap and easy to use.
So, there's a short " Petty Model History/How to" with extra emphasis on the years I figured you'd have the most interest in. I'll get some pics up of my 68 when its done, but this ones a mega project so it might be a few months yet. Hope this is all of use to you and even more inspiring. Long live the King, the greatest race driver of all time.
Dave Bayer