906 or 915 ?

I paid $900 to freshen a pair of iron heads a while back, and I certainly would NOT have done that, except they were professionally ported 286 max heads. I would assume, getting a set of 906s to the same point (reconditioned, but not ported) would run almost the same. I would NOT spend that on 906s.
My guess is the PORTED 915s are pretty good. I'm also guessing that if they are going on an M code A-body, they'll have to be iron. I might be tempted to offer half of what a pair of stealths run, but I think $750 is not bad. You CERTAINLY can't get equivalent performance from your 906 for anywhere near that price. If the previous owner was running a roller, you can expect to have to change the valvesprings.
Only thing I'd worry about is too much compression, depending on your pistons.