If you are thinking about doing it yourself you might spend the 20 or $30 to have the head magnafluxed before you go to the trouble of doing the work on the valves, especially since you've got cracked valves.

If you hunt this site somewhere it'll say which valves are direct swap for the ones in your head, might be BBC or SBC.

Also, if you look on ebay for Chevy 235 you can get a set of NOS valves for fairly cheap.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi