if the lager shaft 'w/reinforcement tubes' 6 blade pump was with a/c in mind, it's due to the extra belt causing more tension and further out on the shaft.
"... further out on the shaft" suggests that different water pumps might stick out farther from the timing cover. Is that what you mean? I just put the newer style aluminum water pump (72+?) on my '65 273. It looks just like the photos (didn't count the blades). Problem is it sticks out ~1" more than the stock cast-iron pump, which prevents me from installing a clutch fan. I will also need to get a later style pulley (deeper) to locate it in the correct plane.
I went with the later style because I could only buy the later style 22" radiators (lower tube on pass. side), so needed the corresponding pump. Also, the aluminum one is lighter, cheaper, and looks better. I didn't imagine a different size.