7 1/4 Why is it so weak?

Parts are scarce for one thing, aftermarket gears or axles, etc are pretty much non existent. It's just not worth the effort to mess with. There are more than a few tech threads about 8.8 Ford rearends which are probably the best bang for the buck as the A body 8.75 housings are pretty expensive. It's almost cheaper to get a B or C body 8.75 and have it shortened.