Look for the crossbolts above the oil pan skirt. There are three on each side. This is the easiest way to quickly ID a 427. Next are the casting numbers on the side of the block however some service blocks did not have casting numbers. As mentioned above the cyl bore is the best way to measure the block. It possible the GT500 had the stock 428 replaced with a 427 at some point. Early 427's ( late 63 early 64 ) had the two bolt motor mounts and 65 they went to the four bolt setup. So crawl under that pup and look for those crossbolts. www.fordfe.com is also a great site for the FE engine. G.