56 chevy/late 70`s firebird

[68c10owner]

so I was talking to one of the lot kids at work and he tells me his dad has a 56 chevy truck thats in great shape but been sitting for years. He also has a late 70`s firebird that got hit in the rear that he picked up to graft the front clip onto the 56. I told him to ask his dad if he wanted to sell it. He told me today his dad is thinking about it. he has a 69 Mach 1 mustang that he`s had since like high school and has been thinking about reviving it latley and could use the money to get it going. I told him I could`nt do anything till tax time so that will give the kid plenty of time to convice his old man that he needs to sell it to me so he can build the mustang with his dad.

Anyway, my question is, are the rearends the right width for the 55-59 chevys? I`m thinking about a set of 15x10s and 295/50/15`s in the back. Probably on some steelies for now. Will the firebird rear leaf springs adapt to the 56 chassis without too much work?