Absolutely it’s worth while to upgrade the engine internals and have the heads PnP’d before the blower. Truthfully though, if 375-400 flywheel horsepower is all you’re looking for, then you’ll easily see that with stock untouched PI heads and about a 7 psi centrifugal set up. And at that low of a horsepower level with the centrifugal huffer, it’s not even really necessary to forge the internal components.
Now….that being said, if you were to beef up the bottom end and add a set of PnP heads, you could take that blower set ups potential into the 500-550 flywheel horsepower level, with very little hassle. If your satisfied with your projected power levels, then you should do well with just a blower and nothing else, but if you’re willing to spend the money on toughening up the engine and adding ported heads, then the sky is nearly the limit.