Cody,
This forum's members are the best around. The members here enjoy what they do and are always willing to help.
A good donor kit for the tubbed rear is the AMT 68 Roadrunner Pro Street. This kit has really large tubs and a nice 4-link and Dana 60. DenconResin.com also sells a resin version of the Roadrunner back half. Swapping out the back half will be easier then scratch building one. There is just some trimming involved. I have a Pro Street chassis from the AMT 70 Coronet that I'll be using the back half in a 68 Camaro 1/4 mile bracket car. I'm using the AMT 68 Camaro kit for this build. I'm going to order the 4-link/Dana rear from Dencon Resin. The AMT Coronet Pro Street has a slightly different back half with narrower frame rails. However there are no wheelie bars like in the Roadrunner kit. The tubs in these 2 kits are huge and these kits come with big Pro Street tires. I think they are Mickey Thompson's.
I hope some of this helps.
Mike.
Here is a picture of the AMT 1/25 70 Coronet Pro Street tubbed chassis.