If the exhaust manifolds are shot, put the thing back up for sale. Even if they're not, you would be $ ahead putting the thing back up for sale. That project boat looks like one big black hole to me. I like boats and the inboard is cool but you have to know when to call it.

To answer your questions, you can't just run the drive in reverse, well not for long anyway. Depending on the drive, they have an internal oil pump that doesn't pump in reverse. Depending on the drive (paragon? velvet?) they can be reconfigured to change the input direction. If you do that, you can basically plunk in a garden variety small block and that may be cheaper than re-configuring a SB to reverse rotation.