A body/B body housngs length?

Either will work. The early B-body isn't that much wider but you do have to relocate the spring perches. You'll also need different wheels with more back spacing. $350 for a 3rd member with a sure grip is cheap enough but as Pete said 4.56 gears are not street friendly gears. I had 4:10's in my Cuda at first and didn't even like them. Hate to imagine 4.56's

If you can get an A-body rear, even without backing plates, your better off as it'll bolt right in and you can use the same wheels you have now. You can find stock size 10x1-3/4 brake stuff for an A-body fairly cheap or you can look around and upgrade to bigger brakes which is a good option. Only disadvantage is you'll need different axles.

As for the Ford 9" rears. There are several cases of them like an 8-3/4. I'm not positive but I don't think all 9" cases even have the gussets so they are not really that strong of a rear. They also have a smaller pinion diameter than even the 741 case 8-3/4. Only reason they do as well as they do is they have a 3rd pinion bearing n the back for support. If it weren't for that they'd be junk, IMO.