the v6 has forged pistons and rods
for the V8, you could get yourself a Performance Automotive Warehouse book and the factory specs for the rods, try to find one that is pretty close, then have a machine shop do the required machining to get them to spec (thickness, off-set grind, etc) That's not all that uncommon in the performance engine building world.
Pistons... there's a number of companies out there that will manufacture slugs to your required dimensions.
The biggest issue you'll run into is the crank and that's the real sticking point. It gets REALLY pricey to have custom cranks made I don't know the journal sizes or anything, so it's hard to say how much a stock crank can handle. But given the size of the engine... I think you'd be seriously pushing it with 400 BHP.
And of course we haven't even dipped into the issue of mains caps. Aluminum block not designed to handle large HP figures. Really it's hard to tell without having one tore down.
Then after all that is said and done, for NA applications, the intake would never suffice; sheetmetal with turbos or a roots type blower would be the best choice. And of course headers would be a problem... Not to mention injectors big enough to supply the monster.
But, if you'd like to send me a 3.9L that I can play with, I'm all game!!!
All said and done, I think if you actually went to tackle building a high HP 3.9L, it would run like a raped ape. BUT it would cost so much you'd be better off strapping it into an F1 car and givin 24hrs at La Mans a shot
And to top all of this off, the standard gearbox would never handle that kind of power; getting a TKO600 (which would be idea for the kind of power I'd assume you're looking to produce wanting forged internals) to work in that car without everything else flipping out... well, if you could do that... I'm VERY impressed. However at that point you'd have so much work and money into it that a 4.6L 4V / T-56 swap would be a better choice.
