i think if you're in that far, it's in for a penny, in for a pound. i understand a shoe-string budget but personally, i think that's kind of false economy on the bearings and rings once you're that deep.
that being said, if everything looks minty and the clearances are acceptable? *shrugs* sure why not. just make sure everything is labeled precisely.
the problem with running an old converter in a freshly built trans is that you're gonna cycle whatever old junk is in there right on thru your new hotness. at the very least find a way to flush it if you can. or most of it.