Both the 427 and 454 had a standard bore of 4.250. They both can be bored .060-inch oversize safely, but you must make sure the bore isnt already at the limit. If it needs to be honed to clean up the bore, youll be exceeding the safe bore size, and hot spots and cylinder-wall cracking could result. Purchasing machined parts should always be on a buyer beware basis. You can never check too many times to make sure the surfaces arent damaged beyond repair. Purchasing a complete, running engine from a well-known source (so its easily located if theres a problem) is always the best way to purchase an engine. Some sellers simply disappear with your money, leaving you with a part that would require more to fix than what a guaranteed good part would have cost. When checking the block make sure the threads arent stripped, check the bearing surfaces to see if the engine had a spun bearing and, in cooler climates, check the freeze plugs to see if the engine has ever been frozen. Questions can have any number of predetermined answers, especially if the seller has had an entire weekend to find out which ones sound best. Taking along tools you know wont lie is the best way to protect yourself. Inside-diameter micrometers and a dial caliper are the best ways to check the bores for wear or excessive boring. Check both at the top of the bore and midway down the bore.