Having lost parts of my original cockpit for the old Monogram P-47D bubbletop kit, I embarked upon a quest to discover if one of the after-market resin pits made for other brands would "fit" inside the fuselage of the MG/Revell bird. I went with one from "True Details", tho the Aires pits really looked good. The price was my consideration, and realizing I would probably only ruin it with my paint job.
First, the model I am using is way old (about 45 years) and in rough shape. She’s a battered old bird, but all the better for a bit of experimenting.
And there’s the nice, shiny new resin cockpit, waiting for me to take a knife to it. Since this is a P-47D bubbletop build, the backplate for the “razor” can be laid aside.
True Details instructions for what to trim on the parts is not exactly clear from their small data sheet. It’s a trial and error sort of method. I eventually ended up cutting and filing away a lot of the joint areas from the sides. This may be what was intended. Eventually you can get a reasonable fit.
After the trimming and filing, the parts are ready to be test fitted. Some white glue is useful for holding parts together while fitting, and can easily be taken apart again. Painter’s tape helps also. Not too bad. The TD cockpit certainly seems larger than the original one.
In the MG/Revell P-47, you will have to cut away the two tabs that hold the normal cockpit in place. The lower one can be left, and it will help support the cockpit. Note also that you can remove the original gun-sight on the MG/Revell fuselage and replace it with the True Details version. Mine was already broken off, so no choice there.
The first test fit, with the seat and control panel in place. It’s not bad, but the TD cockpit does not seem to be high enough. Needs something for the cockpit to sit on while it is being glued.
I did some more trimming and added something to the bottom. These small bars on the sides are not necessary, so I trimmed them away. A useless step I later learned. You don't have to do that get the cockpit to fit.
Second test fitting.
This looks a little better. I am concerned that the cockpit sides do not come up to the edge of the fuselage, but the old cockpit did not either. The small tabs along the side of the MG/Revell kit represent where the cockpit is supposed to rest, and that is exactly where the new TD cockpit also rests. I think this will be as good as I can do. With some paint and a little filler, it will certainly be an improvement over the old one.
Just to make sure, the canopy is placed over the headrest. Nice. Next: To paint the cockpit.
(I must admit, I detailed it a bit more than I thought I would. Even with the help of the Optivisor [thanks for the recommendation!], the work was unsteady. Got anything for hands? 
)
As this picture shows, I had to scratch-build a new instrument panel. I could not for the life of me paint all those dials. Luckily, one of the decal sheets I ordered had an IP decal, so I cut out a new panel from some plastic sheeting and glued the decal on (backing and all). I wanted a cable for the electrical connection to the sight, so I found a metal twist tie from a garbage bag and used the wire from that. A piece of clear plastic from a package became the sighting glass. The gauges should not have white rings around them... but, hey, at least I didn't have to paint them.
Then I silvered the edges and did a black wash for the cockpit areas for oil and dirt. The picture makes it look as if there has been a fire in the cockpit...
Well, it's supposed to look dirty and used. Anything else is just my lack of painting skill.
The fit in the fuselage is not too bad, but I need to come up with a support that will keep the cockpit from sliding down as it is being glued. I intend to carve some foam into a curve and fit it under the cockpit. Those foam packing peanuts will finally have a use!
Engine is starting to be detailed too. Grime is the key...
Luckily I keep a lot of old packing boxes in the closet, so I have some foam (to spare). This piece is about the right size. I used foam because it is simi-rigid, with enough give to push the cockpit up into position, while soft enough to conform.
With a bit of cutting and sanding, you might end up with something that will fit inside the fuselage just at the bottom rear of the cockpit.
Make sure it doesn’t cover up the wing slots in the sides. You’ll need those later. Oh, and if you can find something else to use for support, do it! Otherwise you have to contend with all those tiny particles of foam that seem to want to stick to everything! (But that’s what painters tape is for.)
A preliminary fitting indicates the piece is still not tall enough, so a good application of painters tape helps beef it up.
Now the cockpit sits perfectly. All that is left is to finish up the model and glue the cockpit in place.
A True Details resin cockpit will fit in the old Monogram/Revell kits. Just takes a bit of patience and some spare foam. 
Thanks for looking!
Gary
PS> After gluing the cockpit into place on one side, I removed the foam support as it was no longer necessary. Cockpit was snuggly in position.