Going from memory here...
- MP made B & RB valley covers intended to work w/ the Stage VI heads, but they weren't exactly sophisticated... just stamped aluminum plates w/ the head-side edges turned up where they run into the heads
- I thought those MP covers were supposed to be sandwiched between the optional RB spacers and the heads themselves, which is how I used it on an RB w/ a standard-type RB intake
- Not exactly sure how the B valley cover would seal when using an RB intake that didn't use the spacers, other than a sh!tload of RTV along the edge where it butted up against the heads
- The '337 intake won't do what you're looking for since the integral valley cover of that intake will still sit way above the front & rear edges of the block, unless you fabricate some type of spacers like AndyF shows he made in his book to do something similar when using the '337 intake w/ Edebrock Victor heads
- The dedicated RB Stage VI intakes won't do what you're looking for either and, I suspect, would require cutting off the integral valley cover to even line up the ports

Maybe you can make something like, or possibly modify, the valley covers that either Hughes or 440 Source sell. I'd suggest fabricating your own to allow a wider strip of material on the head-sides of the plate, then drilling & tapping the cover & the heads (like the Victor covers) to enable putting sealant between them and tightening the cover in place better.

Just an afterthought, but maybe you can start w/ the Victor B valley cover and modify that to work w/ the Stage VI heads?
==> http://www.jegs.com/i/Edelbrock/350/7799/10002/-1