2.76 open to 3.55 Sure Grip...expectations too high?

Those pistons are listed as flat-top hypers with no valve reliefs and a compression distance of 1.755. In a 9.600 318, that maths to being down in the hole .064, which at 3.950 bore is 12.95cc. the gasket is gonna be between 6.3 and 8.6cc. And unmilled 302 heads might be 65cc, so that totals 86.75cc, hey that ain't so bad.... with a 9.95 bore your swept is 665.68, making your Scr to be ~8.67.... but this assumes a 9.600 deck.
With the new numbers, The Wallace predicts ~132 psi, and the new V/P is 106, better than 99, but still pretty weak. We'll see how your Compression test compares.