[MrPbody]

Well, actually, Pontiac was second to use the "air gap". Studebaker beat them by two years...
Guys like savagewolf are the reason so many people believe Pontiacs blow up and can't make power. None of the above... Myth and superstition abound, and Pontiac is no exception.
All of what sixtyeightbird and gunstar1 said are completely accurate.
There are many quirks to building any engine family. There's a new book (August, this year) out by SA Designs. Author Jim Hand is an old Pontiac racer from WAY back... He collected some of the better Pontiac "minds" and used their specialties to produce the most comprehensive study of the Injun Engine since the HO Racing book from 1976. it's called "How to Build Max-performance Pontiac V8s". It's available at BOP Enginieering, Performance Years, Ames Performance, Central Virginia Machine Service, Kaufmann Racing Engines, Jim Butler Performance, and probably a few other Pontiac outfits. Chevy guys won't like it. It spells out the history of Pontiac in racing, and the success they've enjoyed over the years. It also documents many of the engineering "firsts" from GM.
A couple other websites of interest:

boyleworks.com/musclecars
classicalpontiac.com

These sites have excellent Q/A sessions and vast data banks of ACCURATE Pontiac information.

As others have said, the compression ratio in that 400, with the SD heads, is woeful. 6.5:1 is probably more accurate. The SD heads, while the last real race head for a street car GM produced, was designed for 8:1 with 455 CID. You can raise the ratio by installing bump-top (domed) pistons. But Pontiacs have a unique combustion chamber design that does not respond well to domed pistons. It inhibits flame propegation badly. Speed Pro does still list the part number for the dome piston, designed specifically to increase the 8:1 to 10:1 in a 455.
Your best bet is to put those SD heads on E-bay or something, and take the proceeds and turn them into a pair of Edelbrock heads with the 72 CC chamber. That will REALLY wake up that 400! SD heads are desireable for high-end racing and restoration. Not much else.
FWIW