[Brentco]

X2 above. The electric fan and lack of fan shroud are suspect. I know you’ve tested with a high speed shop fan, but that might not be enough. Also infrared thermometer on temp sender to verify gauge accuracy.

Some say that air pockets in the cooling system can cause overheating. When I’ve have had similar cooling gremlins with new/rebuilt engines they’ve eventually gone away after tinkering with the system and filling/refilling the coolant, which makes me think there may be something to that. Try filling the system with the heater hose (coming out of the head) disconnected, and then reattach the hose once coolant starts coming out of the hole. Also drill a couple of holes in the thermostat (assuming you’ve put it back on). Then when you start it up, turn the heater on full blast, and leave the radiator cap off, adding coolant as necessary. This should get any air out of the system.

The only other big factor that has worked for me in the past has been adjusting the initial timing. I see you’ve already tried that, but you may consider further investigation down that road, as it seems to have the most significant effect on overheating when everything else is ship shape.