7 1/4?

I have the same issue, only it is my plan to get an 8 1/4 out of an M or F-body Mopar and replacing the leaf spring perches (the existing perches need to be cut off and welded up in another location on the housing). I have noticed that frequently, just because a prospective rear is for an A-body, people want more for it. M and/or F-bodies are FAR more plentiful and the housings are about 1.14" wider from flange to flange. This means it will be about 1/2" wider on each side....a pretty good fit.
I know you're hard up for some $ at the moment, but if you cannot find an 8 1/4 A-body rear at a reasonable cost, the F or M-body option is a good bet. The 7 1/4 will not withstand much punishment, and could fail quite spectacularly. This usually involves loud, unsettling noises and shrapnel. If you MUST use it, be really careful. You're better off leaving the rear suspension unmodified as well. There needs to be some give if that axle is to survive. As another member said earlier, if the tires bite onto the pavement, the rear will be stressed and can break quite suddenly.
When you have some more money, look for a rear out of a van. If you need to shorten a rear to make it fit, then you may as well seek out the sort of vehicle that is extremely common. Parts will usually be less expensive, and it should not cost any more to shorten a van rear than, say, a C-body rear.
Keep in mind, though, that many vans in the 80's used an 8 3/8" rear (oddball size). Some, depending on the year and weight rating, had an 8 1/4 (I believe), some had an 8 3/4, some had the 9 1/4, and some were built with Dana 60's.
If I have given any information here that is not correct, I'm sure someone will point it out. I hope this is of use.