Holley 750 what to look for?

I'm planning to use a 750 double pumper mech secondary on my 360 magnum build per advice from Brian at IMM who will supply RHS heads.

I see a lot of these on EBAY but just not sure exactly what I should be looking for there seem to be a number of variants. I'd like to save some cash and don't mind doing a rebuild, just don't want to buy the wrong thing.

Suggestions?