Welcome to CHR.
I agree with FordGuy, it would be cool to keep the flattie, maybe hop it up a bit and run it. If you need more power then I would probably run the pontiac as something a bit different, considering it is freshly rebuilt and that money has been spent. If neither of those trip your trigger than I would lean to the 390FE, Ford in Ford route. We'll look forward to pictures and progress reports on your build as it goes along.