The initial plan my engine builder and I had discussed was to build a 400/512 using a 440 Source kit with flat top pistons. We intended to use Edelbrock RPM 84 cc heads (which I already have) that will be ported. The plan was to have a compression of somewhere between 10.5 and 10.8 in order to be able to run on pump gas. He told me after talking to someone at 440 Source that we wouldn't be able to use flat tops without milling the heads in order to achieve the targeted compression.
I recommend you find a new engine builder, as BSB67 said, a 4.25 stroke engine with flat tops and 84cc chambers will be over 11:1 and a decent builder would not rely on 440 source to figure out such a basic concept like compression calculation
