You might find some better advice and examples at a place like this: http://www.fullsizechevy.com/forum/g...n/performance/
Not a lot of experience with that motor on this forum, but I do have a little experience with the L31, so I'll offer this:
First, 400 whp means you won't have much left of an "L31". About the only thing I'd keep, with that target in mind, is the shortblock, and then you might as well start with any old SBC Gen 1 block.
Yes, the heads are excellent as far as iron heads go, but to take them from their ~250 whp baseline capability to what you want, your money would be better spent on aftermarket conventional Gen 1 aluminum heads and avoid the Vortec intake manifold limitations. so yes, old-style heads are the way to go if you're serious about that goal.
You're right about the stock injection needing to go, even if you were to stick with the L31 heads. Even the GM upgrade to the poppets will get you nowhere toward your goal.
And for 400 whp.....forget TPI. It's too much of an "rpm choke" to get you near 400 hp without ridiculous money and mods heaped upon it.
If your goal was more like 300 rwhp, then you could make it happen with the Vortec heads and modifications to the manifold. Check out the forum I linked above and search under vortecstroker screen name. The stock manifold quits about 5500 rpm, so figure 325whp as the limit with a 383, and 300 whp with a 350/355. That cam you linked would easily get you to those numbers, but may be a stretch for 400 whp.
Edit: There are also a few threads on this forum in the "Tech" and "Engine Swap" sections on L31 buildups: http://www.thirdgen.org/techboard