I'm with Steve on that one, particularly after seeing sets of #13's and #62's out there from between $200 (cores) to $500 (rebuilt and running).

Converting #15's to large valve, HD seats, screw-in studs, 6-bolt flanges, etc will run double that amount (with rebuild). And that's after you pay for the cores.

If you insist on converting small-valve heads, there are #5C heads which have the large intake valve, hardened seats and 6 exhaust flange holes already provided (from the factory).

There was a set on eBay last night that will probably sell for less than $200.