Are you sure that 400 motor has 9:1 CR with 6X-8 heads?? The 6X-8 heads have a pretty big combustion chamber and even with flat top pistons and the milling, I'd be shocked if you are truly at 9:1.

I've always thought that the Pontiac 350 was a very underrated engine. Years ago, I had a 71 Lemans Sport convertible with a 350, 4bbl, and a set of 3:23 gears. That car ran strong. The 350 had great street manners and pulled hard through the gears. I beat quite a few 5.0 Mustangs in some stoplight racing. Of course the hot rodder in me wanted more and the stock 350 was replaced with a mild 455 but that's a story for another day.

The Summit 2801 is very similar to the factory 068 grind cam which is a great street cam. I'd say that 400 would run strong in your car. If you're looking for a little more oompf then drop it in and hold onto the 350.