On 28 February 2014, Paul M Dean in Fife, UK wrote this:

"I am in the process of fitting my newly rebuilt B 3 bearing engine in my 1500 A and hit a bit of an issue about the water pump which I have now solved (hopefully!).

"My relatively new A pump would not fit the B block. The B block uses the same bolt fixings but it also has a locating dowel set into the block. I then purchased a new early B pump from Moss but I couldn't use that easily either for 2 reasons. The first reason is that although the A pulley would fit the belt was out of line with the pulley on the crankshaft. This problem could almost certainly have been fixed by buying the B pulley to match the B pump. The second reason was that the body of the pump protruded about 13mm (1/2 inch) further forward, hence once fan fitted fouling the radiator. This problem could probably have been solved by moving the radiator forward but this has other potential issues.

"As well as the difference of the dowel in the block and the hole to accommodate it in the pump casting, the block casting is somewhat different in detail, the A one has a sort of flange as part of the casting that is missing from the B one. Also the B impeller goes slightly further into the block and the B pump body lacks a raised area to match the flange present on the A block. This is very esoteric and I suspect the B set up is an improved version but functionally in only a minor way.

"I have solved my problem by slightly filing the body of my A pump so it doesn't foul the dowel in the B block and hence fits fine".

Seems like you could remove the dowel from the B block and not have to modify the A pump.

The MGA pump has an extended pilot diameter on the pump body casting which extends into the engine block for alignment. The MGB pump has a flush rear surface with recessed impeller, then using the dowel pin to assist alignment.