I agree that it is probably not the pump. The pump is a positive displacement type, so if anything stops the flow, it will build up tremendous pressure and stop turning, i.e. your symptoms. You should have continuous flow thru the spool valve on top of the steering gear. The valve just proportions the pressure between the two sides of the piston, so the piston follows your steering input, similar to how a brake booster works. I think the problem is there. Either the spool is jammed, or something is clogging your hoses and tubes. It is easy to remove the spool valve with the gear in the car. I did that in my tight early-A w/ V-8, and you have inches more in your
B-body. Also check the return fitting since most (all?) have a spring-loaded check ball in there that could block flow