Honda-SOHC

hi im currently building a cr 750 replica and have a problem with the head i bought an f2 head off ebay and sent it to a tuning company after porting etc they sent head to get the 5mm valve kit fitted i got from cycle x but said the valves are to big after a bit of resurch i think theres two f2heads one with smaller valves with i think i must off purchased is there a way of telling ie casting numbers before buying another head as ive got to have it tuned again before fitting of the race valve kit i think the inlets on the race kit were 34mm thanks matt