A completely stock, fresh EFI 351W can handle 8-9psi to 5000 RPM without a problem, assuming you're running 92 or 93 octane. Less boost if you're running lower octane because the compression ratio obviously is fixed.

Some people have done a little higher than 8-9psi, others got close to it and couldn't quite reach it without serious pinging.

If the engine is old, worn out, and in need of a rebuild, adding boost will just hammer the already loose bearings and knock it to death.

What it can do on propane, I'm not sure, never tried. I believe you would get better results with a higher compression ratio than with gasoline.

I got a diesel snowblower engine to run on propane, and that's something like 15:1 or 16:1 I think...