FWIW, I think Trans Am Jim and Cobra Ed are on the correct path. If you have your timing set at 28-30 degrees at 1200 rpm, there's something wrong with your timing. Mr. Parham at Southern Automotive impressed upon me that ignition timing differences of just a few degrees will be evident in engine coolant temps. Check out this thread on the FE Forum:
FE timing
Most of us are running 9-10 degrees at idle speeds of about 900-950 rpm (this is with the mechanical advance MSD type distributors). Cobra Ed has identified one possible cause: the ring around the harmonic damper may have slipped. This is not unheard of with 35 year old dampers.
I assume this is a relatively new problem, as I saw your name listed as a Run'n'Gun participant in KitCar magazine and I assume you didn't have these problems then. Grasping at straws myself, the only other possibility I can think of is some sort of valve timing problem. Is it possible you've jumped a tooth on your camshaft timing sprocket? This could possibly account for some of the backfire/belching smoke problems you've cited. If the valve timing is off, it may require that you would have to compensate with altered ignition timing to eliminate backfire and such. The altered ignition timing would then lead to overheating. This is a long shot, as I would expect you would also experience a noticeable drop in engine performance at the same time.
As far as low coolant flow at idle, in my observation, even at operating temperature, there is not very much coolant flow evident through the expansion tank at idle rpm with the stock-type water pumps and pulleys.
I wish you luck, big guy. Keep us posted.