Overheating issues

[Ken/Alabama]

I’ve got the newly rebuilt flathead back in my 40 and it’s running good . Problem is it’s like the farther you go the hotter it gets. With thermostats or without, block is clean, new radiator, rebuilt water pumps , timing is good. Yesterday I removed the upper radiator hoses and filled the heads with water up to the neck and removed the belt so the pumps wouldn’t turn. Cranked it up and looked for bubbles in the heads, there wasn’t anything there so that eliminates a leaky gasket . Before I got this car it sat in an open shed for the probably the last 20 years. I’m beginning to wonder if the exhaust could be partially blocked by mice or something. I’m at wits end with this thing.