The C4-G heads are 64-65 352/390 heads. And as snakeherder mentioned the block could be one of many with that casting number. The date codes will at least tell you if the block and heads are original. Short of taking a head off to measure the bore it's hard to tell. The stroke you can measure easily enough to get a better idea what your dealing with. The 352 had a 3.50 stroke and the 390 a 3.78. The 428 a 3.98. I'd start with checking the stroke as you don't have to tear into anything to check it. G.