The power pack heads from 57 and the engines they were used on were rated at 265/270 hp, I don't remember which, and the 283 hp heads came off fuelie engines and are rare as hens teeth. To retain the valve covers you have you will have to find heads from 57 or earlier. If your engine is stone stock it will be running a solid lifter cam. At one time I ran the fuelie cam from 63 in my coupe and the valve lash was 8 on the intake and 12 on the exhaust and it would spin higher than 7 grand as I bent some push rods one night in Oceanside when I'd had one too many.