I don't believe the rear axle is too wide. I put a 80 Chevy 1/2 ton 4x4 rear axle in my 51,only an inch or so wider than the original, I bet the 70 something axle is the same width as the 80 axle.
The small block will fit, you have jimmy the steering box over a touch,the stock exhaust manifold is a tight fit on left side.Some use a right side mani rear dump on the left,some say a rams horn works, a little trial and error.
If ya drive hard,it's best to add discs in the front,the stock drums are marginal for fast driving.
