I'm running a SFT with .490 at the cam .462 at the valve. The cylnder heads are later oem castings, and oem valve guides. When I checked the retainer to guide clearance there is a little more lift available, say .510, but the heads have umbrella valve stem seals... With the right retainers (Crower 87049) and +.050 keepers you should be able to get a .557 SFT in there.