Unfortunately, the 624 casting are probably the worst of the GM castings for cracking. They are prone to cracking across the exhaust seats and thru to the heat crossover. They’re not good candidates for any decking either.

Mortec says they are from a 75-86 350 or 400, 76cc chamber. They were manufactured with 1.72/1.5, 1.94/1.5 or 2.02/1.6 valves

If they have the 1.72/1.5 or the 2.02/1.6 valves and they are perfect – they may be worth drilling steam holes. If they are the small valves or (as is very likely) show any cracks, I’d start looking for new heads.

If you have any budget, the Vortecs are a good way to go. This means a new intake manifold as well, but will definitely bring the 400 alive. GM Part Number 25534421 ($1100 a set or less on ebay) is a great head, 185cc runners and 66cc chambers. They’ll need steam holes drilled.

Good Luck!
Glenn