I am running a 75 Trans Am rear axle in my 37. I also used 2" wider rear fenders. I don't think I needed the fenders wider but it does look better to me. I have seem some trucks with the tire right at the edge of the fender, I like them to fit under it a little. The axle width was the same as I took out. It had a 55 Chevy axle under when I bought the truck. I just cut off the spring perches and welded them to the new TA axle.
You can us a 70-78 Firebird/Camero axle, some even had rear disk if you can find one. I had mine mounted and bed on before I figured out the tires weren't centered in the fender. I never thought to move axle back, I allready had the driveshaft made. I ended up cutting the running boards back about 3/4" and moving the fenders forward.
The above post is right on, on the angles. Its really easy to do using a cheap angle finder. Try Harbor frieght, they have one that works OK for little money.
Talk to the brake people about what master cylinder to use. I ended up with a 68-72 Mustage manual disk/drum unit. It has a 1" bore. I had to install a adjustable proportioning valve in the rear brake line to keep from locking up rear tires to easy. I also have the Camero type disk's up front. Joe
