Big block running hot. Can’t figure out the issue

[Laqota]

So, I got my truck put back together after doing a mild refresh on my both block and I can’t figure out why it’s running hot. Here are the details:
- mark iv 454
- gen 6 pistons and rods with new cast iron rings and trimetal bearings
- gen 6 roller cam with new roller lifters, pushrods, guide plates, etc.
- 781 heads
- edelbrock dual plane intake and q jet
- Converted to serpentine setup with new reverse rotation water pump and fan clutch
- timing is at ~12 degrees advanced and vacuum and afr readings are normal (~16 in at 6-700 rpm idle and rich idle with leaner cruise)

The engine starts and runs great, but will get up to ~240-250 and stay there. When I shut the truck off the upper rad hose is rock hard, but I ran a combustion leak test and the fluid never changes color. Tried it for like 30 min before giving up. It’s not consuming coolant as the level stays the same and I’m not “making more oil” and there’s no smoke out of the tailpipe indicating burning coolant. The fan is definitely pulling air through the rad and I ended up throwing a 160 degree thermostat in there but that didn’t change anything. Should I just bite the bullet and change head gaskets or is there something else I’m possibly missing?

drain your radiator and fill it with water (preferably distilled but tap will do just as good). Then do a funny and take out the thermostat and unhook the tube from the radiator that comes from the housing. Put it in to a bucket and start the car. If no water is coming out (with a sink amount of force) after a few seconds your pump might be cooked.

Also don't trust those head gasket testers. Their only good if you have a major head gasket leak. If you have a slight leak that's just barely letting coolant in it might not catch it but even at that wouldn't cause exessive overheating. Go smell your exhaust, if it smells sweet and potent then you've got yourself a head gasket or an intake gasket failure (if water cooled intake)

Also are you using a 50:50 mix or close of coolant and water? You can't just run either or because it will overheat.