Just saying the heads are new does not mean they are good. I recently rebuilt a C15 cat engine and they head that was delivered from cat was cracked. Also, this could be caused by head gaskets installed upside down. Or cross threaded head bolts or the heads could have been torqued incorrectly and that could have cracked them. Never overlook the little things while diagnosing, it'll come back to bite you.
Vehicle: '99 Dakota SLT V8, Club Cab, 5 Speed, 4x4
Location: Okemos, MI
IMHO, I would start with the assumption that it is the head gaskets. You have a pretty serious problem and I would start with the cheapest potential issue. Whats the worst that happens if you pull of the heads and the gasket isn't wet? Your out a set of bolts and your time. But, you were going to take the new heads off if the block was cracked anyhow, so you really haven't lost anything...