I tried getting a couple of sets about two years ago for some tractor pulling guys.
There were none to be had, and at the time, Russ told me they were having a hard time finding a foundry that was willing to do a small run of them(like 20 heads).
Apparently, they are not fast movers for Indy.

As far as the cracks go, have them pressure tested. If they don't leak, just keep running them.
In some of the oval track classes, Dart iron SBC heads crack between the seats very often. The guys just keep running them........ No problems.