Mtschill,
First off, welcome to the jungle...err...ship forum!
You have a couple of options actually...depending on your budget and skillset.
Cassin Young is a square bridge Fletcher...the Tamiya kit is round bridge...and there's a slew of differences between the two.
You can purchase a resin kit of the USS Johnston, a square bridge Fletcher, and build her basically out of the box, you'll need to change her camo pattern if you want to portray CY in her 1944-early '45 configuration. Here's a link to the Yankee Modelworks site, that produces the kit. Cost is $120.00, but includes all the photoetch parts you need. Basically, you don't need to buy anything else.
http://www.yankeemodelworks.com/dd557.htm
Your second option is to purchase the Tamiya kit, and the Tom's Modelworks Square Bridge Fletcher conversion kit. It provides you with resin replacement parts to modify the ship into a late war version. You can purchase the set with or without the accompanying photoetch set. Be advised though, that if you choose this option, you really should replace the kit turrets, as they are too small for the kit, have the improper shape, and sit too far forward on the mounts. I would recommend any one of the three sets of replacement turrets, White Ensign, Corsair Armada, or L'Arsenal. I've used all three and liked Corsair Armada's the best.
There are other options you can explore for either kit, if you really want to go 'all out' on this ship. L'Arsenal makes incredible 40mm Mk1 mounts, as well as 20mm single mounts, Mk 51 Fire Control Directors, and other various small bits. These can replace both weapons in either set, as they are of superior quality to the white metal castings used by Yankee or Tom's.
If you have any questions, please feel free to email me or post up here.
The conversion route with the Tom's set will be cheaper than the resin kit unless you decide to replace the turrets, then it will probably be very close in cost.
Jeff Herne