The 526's are the best you are going to do with stock type cast pistons. With a compression height of 1.741", the pistons are still going to be .080" in the hole; you just lose the eyebrows which helps CR a bit. With your McCord head gaskets, you are at 8.45/6.8 SCR/DCR, and with the 1121G's you are at 8.7/7.0 SCR/DCR. A little better.....
Going to KB167 pistons (advertised at $321 USD from Campbell Enterprises in VA), the pistons are now about .010"-.012" in the hole, and your SCR/DCR with the McCord head gaskets is 9.28/7.8, and is 9.55/8.0 with the 1121G head gaskets.
With the 1121G gaskets, you now have an effective quench gap, with the pistons about .038 to .040" from the heads' closed area. (You would have to carefully measure that gap but you are good at that.) With the McCord gaskets, the quench gap is going reach around .050" and will start becoming less effective at reducing the tendency to detonate.
The KB's are going to cure your weak low end torque problems... even with that cam that you have. Did you machinist have another hi-comp piston PN to suggest?
Moly rings for sure..... make life easy on yourself.
Does your machinist know that your cam already has 4 degrees of ground-in advance if installed dot-to-dot (assuming all the parts are machined accurately)? If you're stuck with the the lower comp pistons, then by all means do 2-4 degrees of added advance.