I have a bit of experience with vortec engines. I'm on my second vortec engine build for my car. What is your budget? These heads will make 400 HP pretty easy w/ the right cam and intake and they will do it super cheaply too!
They are great heads! They do require some machine work to do get higher lift. There is not much you can do for cam swaps without pulling the heads and modifying them for more lift. The factory L31 was rated at 255 hp IIRC, but the 330 HP Gm crate engine is basically the same engine, without exhaust and such. Taking off the stock intake manifold and exhaust and cats and replaceing the exhaust with decent headers and some free-flowing mufflers will make a huge HP diff and fuel economy diff.
Really in my opinion the best thing to do is pull the head and machine it for more lift. Then you open up your cam choices. I bought the Comp Cam cutters to cut down my valve spring bosses (pn 4721) and used an 11/32 arbor. Turned out nice, now I have clearance for my Comp xe268h. On my last engine I didn't cut the heads at all and I only put a 255DEH comp cam in it, because I wanted to make sure I'd have plenty of clearance (didnt measure). If youre sticking with a roller cam block (mine was truck block without roller cam provisions), then thats sweet. All my experience is with flat-tappets tho. I'm sure you could hit your power goal even easier with a roller.
My last build was bone-stock vortec non-roller 350 engine, with a performer manifold, quadrajet, and a 255DEH w/ headers and exhaust. Even with this really mild cam I was able to pull a 14.0 with a 2.2 60ft at 100mph in a 3200 lb car, shifting at 5300 RPM. estimated CR was about 9:1. Got excellent fuel economy (for a carb v8) and idle completely smooth, excellent city driver. Not 400hp though.
From what I've read on the internet, a reliable 400-hp ish build is something along the lines of a Lunati 60103 or xe268h and a Performer RPM intake. More lift is better, up to about .550 inch. I'm mimicing this build right now, with a xe268h. There are tons of articles on line about simple vortec head mods that work. More exhaust duration on the cam is good (ie no single-pattern cams). I've read a few results online where 1.6 ratio rockers on the intake picked up a few HP. Performer RPM intake always seems to outperform the Performer, but if youre using a quadrajet then the Performer RPM doesn't come with the right flange, youll have to use an adapter. A spacer on top of a Performer manifold will recoup some of the difference in power between a Performer and RPM, and youll still have the proper mounting flange for a q-jet. I just did a minor bowl- blend on my vortecs and cut down the guides, that's it. My last motor seemed quite happy and smooth to rev to 5500 RPM, it just didn't make any power up there.
Are you planning on putting this in a g-body? If so, you'll have to do an electric fuel pump conversion, most L31s dont have provision for a mechanical pump. I grabbed an '95 TBI short block (similar to the L31s) so that I could still use a mechanical fuel pump. Most of the TBI engines still had the pushrod hole drilled for a mechanical fuel pump. I dont think the stock L31 cam even has a fuel pump lobe on it. Someone else will have to say.
