Your talking about a Dana 61 which is junk in most cases. Basically it was designed for the lowest gear being a 4:10 as it has a different offset. You wouldn't find a lower gear set to work unless you actually made a ring gear spacer as lower gears have a smaller pinion and would needthe shim to compensate. You would have to get a thick gear as well most likely. Off the top of my head I can't remember what the offset is but the standard 60 is .625, 5/8". It is easily mistaken from a d70 by looks as it does have big d70 axle tubes and your correct on bigger spindles but it still has weak 30 spline axles . My opinion get a SRW d70 as it has 35 spline axles and is very strong. It shares alot of parts with the 60 like carrier bearings(unless a hd 70) diff covers,and yoke spline counts. Some dually rear d70 inner axles can be swapped as well for they have the same lengths and are good spares in case.

Corporate 14 bolt chev rear ends are a very strong reared as well and are cheap to build.
Basicallyif you wanna smash on it all day with no problems get a 14 bolt or 70.
But you will next need a 1ton all 1350 driveshaft as well as that's now your weak link.