Check the compression distance. The Diamond piston you linked to has a 1.32" CD, This would be for the 3.75" stroke 6.768" rod engine.

FWIW, those pistons in a stock stroke / rod 400 with would have the piston sitting 0.612" below the deck, and with 84 cc heads new a whole 4.24:1 compression ratio.

Depending on the head volume you are using, stock flat top 400 pistons are already a low compression design.

The stock replacement flat tops have a compression height of 1.812" and the piston sits 0.120" below the deck at TDC. with 84 cc heads and 0.040" head gasket the compression ratio is 7.75:1.

For the stock stroke/rod combination there is not much of a selection of off the shelf pistons. For what you are doing, you will likely need to order custom pistons.