If it has the sand scratch "A" I'd be pretty sure it's a 428. But as Duane pointed out the 66 did not get the 428. It might not have been the original engine then as I cannot remember when the foundry introduced the "A and C" on the blocks. It may have been in late 67 early 68 as the CJ was developed to tell the blocks apart on the assembly line. Before that it was just the std 428 and needed no "special" marks to tell the blocks apart. So you might be looking at a later block possibly. Duane, very good point. G.