I am no 216 (or any other) water pump expert, but it looks to me like there is a lot of open space between the edge of the vanes of the pump and the bolt-on back plate of the pump. That could cause a lot of the coolant to just be running around in circles inside the pump instead of circulating through the engine & radiator.

Any comments from those more expert than I?

'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.