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.
Then you need to get different parts or talk to someone that has a clue as to what they are talking about .
What stroke crank are you using and what is the CH of the piston in the kit?
The most common piston is 1.320 and it works with multiple stroke cranks, and it's close to zero deck in most builds.
you might actuially need a D shape dish piston to keep the compression ratio in check ... 12cc's out the heads
