Your carburetor is lean. It is contributing to your overheating. A/C water pump is supposed to decrease water flow.
1. The crank pulley on A/C cars is always very large diameter and WP pulley is usually smaller than no A/C this makes the pump and fan spin faster.
2. So the pump is actually lower volume to circulate water slower allowing it time to cool off in larger radiator that is sitting behind the condenser and it's added heat.
3. I call BS on this because they all look the same and usually have same vane count. Result of being a engine that is out of production for 40+ years.( I know the anti-cavitation plate pumps look different, save your breath. Gerry isn't using one of these high rpm pumps)
Back to lean condition. Check plugs and try running it with choke partially closed and see if it improves. Both are free checks and free is good.
If the system pressurizes within a minute from dead cold/morning you may be looking at a head gasket.
Still leaning toward lean condition. Are we 100% sure vacuum advance is working properly? As in rate, amount and diaphragm that does not leak.
Also a 30 gallon grease barrel in plastic or metal is a nice diameter for fan shroud. Adjust the depth/length by cutting it lower down the body. Attach it to a flat piece of sheet metal with a number of tabs left in place when cutting the hole in center out. Make them long enough to get past the top ring/lip of the barrel and screw or rivet them to attach. Fold over a 1/8"-1/4" @ 45-90° on the top and bottom to give it some strength and screw it on to shroud tabs on the radiator sides
I believe you've HIT THE NAIL ON THE HEAD BRO! I can easily run around w the choke slightly closed too, AND EVEN OPEN THE IDLE SCREWS A TAD MORE, SINCE IN TRAFFIC, ONE IDLES PLENTY. I think Ray did good work on vac advance and distributor. I can measure the advance tonight, to be sure. I retarded the timing down to 7.5 degrees BTC, which eliminated WOT backfire, though acceleration still clearly fuel starved. I think its time to visit my carb guru..... I have a Carter B & B which needs a competent rebuild OR can go with the Edelbrock 1405. I have a nice intake for that too....
I also have my 1965 Imperial fan shroud. I lack the stop nuts for the radiator, but should be able to find some, and can use metal "plumbers tape" to secure the shroud into the radiator. Excepting the holes, this shroud fits perfectly.