Brodix IK head water passages

Look at a head gasket, you might discover that there would be basically zero flow there anyway.

There's pretty much no heat there... the hottest place in the head, BY FAR, is #2. About 40% of the total water flow needs to go there. About 20% each needs to go through the holes in the corners next to those exhaust ports. The remaining 20% can get split up among the entire rest of the passages. Newer head gaskets usually have 3 large holes at 2, 1 large hole at each of the places I just mentioned, and very small holes at all of the other places.

In fact, if #2 obstructs any of the 3 holes in your gaskets, you should enlarge it. But don't worry about 4, 5, & 6. They don't really do anything except "leak" water from the cold side of the system to the hot side, bypassing the place where heat flows into the system (exh ports), and maybe bleed off air & steam bubbles.

I don't know why it would suddenly seem to need a different thermostat. That doesn't make alot of sense, unless something has failed in the meantime, such as the pump. Not impossible.

But as far as pump pressure... you could measure that at basically any cooling system plug between the pump and the stat; the hot side heater hose fitting (in the intake, not the one in the pump), plugs in the heads themselves, etc. A gauge that would measure fuel pressure would do. Or, you could use a marine one, such as outboard motors sometimes have.