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

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.