He
> ended up changing it out and putting the 8.8 in, just to be safe. So I
> think it would hold a mild 351, versus a strong 429.

It will hold for a while, but it will go through a slow agonizing death. I
put a 351W into my old 79 Stang years back and left the stock C-4 auto
tranny and 7.5 rear end in. Eventually the 7.5 rear blew and the new owners
out in a 8.8 which is a direct bolt in. As others have said- it will be
easier to just get a 5.0L car to begin with and then put in the 351W. I had
an 81 I put a 351W into and we replaced more parts on that car then I car to
mention. Its not worth it. Just get a 5.0L.