1. A 53 1/2 ton could have had the sm420 granny box or the 3 speed on the column.
2. 54 engine should be a 235, can be determined by checking the numbers.
Inliners.org has a good tech page on these.
3. Possible, but I doubt it. The cam will have more effect on vacuum. Get a vacuum gauge for a few bucks, excellent low dollar diagnostic tool. Instructions are at
Vacuum gauge tuning .
4. The exhaust noise you're hearing is the nature of the beast. GM sixes are known for a blap-blap-blap on the opposite of acceleration. Unburnt fuel is ignited in the exhaust manifold.
