Cam/Head Swap Disaster.......

No pressure will build in 45 seconds to 1 minute of running in the cooling system. So any pressure would have to come from the oil system but that one seems hard to do, unless there is a crack between oil system and cooling jacket.

So if there are leaks, then it is more likely the gaskets, or something in the surfaces is not right. But the latter seems unlikely if the heads and blocks were machined. Did you use standard head bolt torque and tighten them in 2-3 stages?