273 302 318 340 build

Some of the 318 truck engines had that low 1.672" piston compression height to push the compression ratio way down to avoid detonation when hauling big loads on bad gas.

TMM is right; the amount you mill off the heads will mostly determine the pushrod issue. With the adjustable rockers, I doubt you will have to worry over it if you don't go wild on milling.

If you used the 302 heads at 64 cc, .028" thick head gasket and those flat top TRW pistons, then your static CR will be 7.6. Install an E-4 cam at an ICL of 102 (6 degrees of cam advance, then the dynamic CR will be 6.6.... expect poorer than /6 torque at low RPM's.

Mill the heads with the above combination by about .040"-045" to get to 57 cc chambers and then you are at a SCR/DCR of 8.1/7.0 Not great for low rpm torque.

Then change to the cam to a Racer Brown or Crane 248 degree hydraulic and set the ICL to 104 (6 or 8 degrees cam advance), and the SCR/DCR is 8.1/7.2. Still not there; not as good as a stock 5.2L Magnum

These pistons 'in the hole' are putting your low end torque 'in the hole'. Keep the cam as small as you can stand if getting rid of the low end lack of torque is still the objective. The only thing helping will be the added cubes.