Getting 350-400 hp from a 400 with ported 6x-4's is possible, but using an existing shortblock with unknown specs makes reaching your hp goal more difficult. It all depends on how well the short block was built- quality short blocks have file fit rings with consistent ring gaps, zero to .010" piston-to- deck clearance, blueprinted oil clearances, decked and align honed main saddle, and a balanced rotating assembly. Unless you know all of these steps were taken and know the shortblock specs you won't even know if the pistons and rings will adequately seal the added compression from milled and ported heads, or if the block has good oil clearances. If you chance it and add ported/milled heads and a cam to the existing shortblock, there's a possibility the performance will fade due to shortcuts in machine work and/or cheap internals.
There are so many variables involved in an engine build to get a specific hp output- the builder of your engine needs to pick the cam, set the SCR, and pick the right internals. You should ask him what he thinks is the best cam for the job- if he doesn't know, you should find a builder that does and has a reputation for good work, as well as has references. If he can't port the heads, have them done by a porter who knows how to do Pontiac heads- the current Pontiac books on Pontiac engine building are good reference guides.
Geno
