Remove the radiator
Remove the fan and pulley, bottom hose and alternator where it connects to the pump.
Remove the pump - unless you have been able to drain the block more coolant will drain out.
Check this bypass hole goes up to the bottom of the thermostat ...
... and this one goes up into the head. Use stiff wire and compare the amount you can insert with the outside afterwards.
New pump installed, ready to refit the pulley, fan, radiator and bottom hose.
The 'original' bolts as removed (all UNF), barely 1/4" engagement with the block, despite the drilling being about an inch deep.
The originals (left) are supposed to be 1 3/4" and 1", but are about 1/8" shorter than that. The new ones I bought based on confusing and incorrect information on the MGOC web site (since corrected following notification) are way longer, even though they are only supposed to be 1/8" longer.
As fitted to the pump A is the long bolt and slightly too long, B is one of the short screws and way too long. C is an original short screw for comparison.
After cutting 1/8" off the long bolt and 5/16" of the short screws, and with washers under the heads, there is more engagement but still well clear or bottoming.
Avoid pumps with pressed impellers as on the right which are less efficient, also plastic ones which can crack and spin on the shaft at high rpms: (Peter Basher)