"...my machinist claimed that the silvolite pistons will help bring the compression down around 9:1..."
Those "8-eyebrow" 400 cast rebuilder pistons are said to sit well down in the cyl, since they have to stock 1.7" pin height. That, along with all the sharp ridges of the valve reliefs, will most likely cause detonation.
Most well known Pontiac engine builders will not use those pistons. The base 400 Pontiac piston is the forged SP L2262F, in the bore size you need. These have only 4 valve reliefs, very similar to the factory pistons, but have a 1.714 pin height, which will raise the piston top, in the cyl, by .014. An Ebay store usually has the best price, usually less than $350--or less than $400, with molly rings.
Better, lighter, forged pistons are available for about $500.
The #15 heads are said to have 87cc chambers. @ 87cc, with flat top pistons, a .030 over 400 Pontiac will have about 9:1 compression. I entered these numbers into the Wallace CR calculator, & came up with 8.99:1 CR.
400 + .030, 87cc heads, 6.7cc valve reliefs, .039 x 4.3 head gaskets, .005 deck height CR = 8.99
The Summit 2801 would be a decent cam for this engine, and about the cheapest available. If you want slightly more cam, a Lunati #10510312 might work for you.
