Most older diff. ratios result in slow and slower results. As stated above, get a whole rear axle. Matching up bolt patterns becomes difficult. It is not productive to have to carry 2 spares, so, to match bolt patterns, if you have 5/10 on 7.25", Dana 70's and 80's come with that bolt pattern in 3500 HD trucks and others. 6 on 8.75"(I believe) come in some Dana 80's in airport busses and others. Some have disc brakes, so, need to work out park brake issues.
Ed
I know there was a post where the whole gear carrier
from a later truck would fit into rear housing
of an earlier truck making the process easier.
Though I don't remember the thread at the moment.
However to do all those things you pointed out
I'd rather stay with the stock gears.
My son does not need or want to do 70 - 75 MPH
and he is not looking to raise MPG.
