My experience with the Vortec heads on the 6.2L vortec EFI intake manifold tell me that BobL statements are pretty much spot on! The Vortec heads with a some nice valve, valve seat and bowl work will flow just fine and easily support 400-450Hp in a 377 inch small block. That Vortec intake on the other had is not gonna get it done right and that camshaft spec. could be improved a little for an overall easy 400-425HP 6.2L with the help of a better exhaust also.
My suggestions:
1. Rework the Vortec heads as described with some good springs, locks, retainers and seals.
2. Raise the compression ratio about 1/2 a point with a about .010" milled of the heads and a good MLS stainless head gasket ,Cometic prefferably that is .010" less thick than stock.
3. Use a camshaft with about 222 @.050 INT.-230 @50 EX. with valve lifts of about .520" and make sure to use a 116LSA for the EFI. Obviously don't forget to clay the pistons and verify valve to piston clearance on mockup.
4. Put on a decent exhaust header like the small block CMI's.
5. Use a new EFI intake like Edelbrocks Pro-Flo XT #7138 with their fuel rails and lines. Your throttlebody will bolt right up along with your injectors and harness should be easily fitted with a little re-route along with sensors and such. I think your stock flame arrestor might even clear and bolt on.
This combo coupled with a mild reprogram of your ECM should make an easy 425HP at 5200 rpms and should work nicely on your stock block.
This isn't a back yard $2k cheap project but for an addtional reliable 90-100 horsepower or so its way less expensive than whole new 425HP small blocks or big blocks for that matter!

Hope these ideas and spec. helps with your plans.

Best Regards,
Ray @ Raylar

Ray, what about the injectors themselves or the ECM. If I still have my boat next winter (rather than something else) might repower or rebuild and pro -charge. Engines run fine but would not feel comfortable doing mods to 400 hr engines.

The stock injectors will fit into the new Edelbrock fuel rails and will flow enough to support the 425HP with a decent duty cycle and the ECM can be reprogrammed on fuel and spark to accomodate the changes.

Yes , raw water cooled stock 400 hour small blocks will probably be needing new internals, so put in forged pistons, H-beam rods, rework your Vortec heads, clamp on a good Whipple supercharger with intercooler and upgrade exhaust and make an easy 600HP per engine and practice breaking drives??!! If you're gonna do'em, do'em right!

You're never gonna be happy until you have almost more power than you need ! Go Big on Power or Go Home!

Best Regards,
Ray @ Raylar

HP was 595 @ 6,000 Max Tq. 576 @ 5000 Tq curve is almost flat from 3,000-5,300 didn't dyno below 3,000 but at 3,000 RPM's it's making 331 hp and 574 ft-lbs of Tq, when full boost comes in @ around 4,800-4,900 it literally pins you back in the seat, the thing accelerates faster than a corvette from 50-85 it happens in a heartbeat!! Not for the faint of heart for sure!

Love it! Nice job.:drool1:

We've had multiple people build these engines or start with 350 mags and stroke, etc. With stock intake, heavily ported heads, slight bump in compression, larger cam, exhaust system, pcm recal, 91-octane makes around 420-440hp. At that point, the throttle body had some vacuum. Its pretty common.