Here's some info on the heads by another contributor....

"The 624 heads are among the worst of the worst. Don't bother spending any money on them. As already referred to, they are highly prone to cracking in critical places, and do not take kindly to port work as a result; their combustion chambers are a cavernous 76cc, and leave you with a compression ratio way short of anything resembling performance; and their flow..... well, they're part of the formula for building 180 HP 350s and 185 HP 400s because they're so restrictive."