Even though it's a B-body, the rear axle in that model is wider than the later flanged B-body axles. Very close to the same as C-body axles if I recall correctly.
The C-body rears are the easiest of all to find and, if complete, come with bigger rear brakes. The 741 centre section is not a weakness at all, nor is it typically for A-bodies with smaller engines. Virtually every C-body sold in Australia, for instance, came with a 741. The gears are just as strong, the only down-sized component is the front bearing on the pinion shaft.
I think the advice you've been give, run it and see how it goes, is good advice. Spend your time and money on things which will make a tangible difference.