First off, I would figure out how far off you are with the big block pump. Put a straight edge across your crank pulley and see what the offset is. Let's say it's 1/2" for argument's sake. I would then get/make some 1/2 inch spacers to sandwich in between the front bracket and water pump, along with 1/2" longer bolts. Then add a couple more between the rear bracket and the back of the P.S. pump, effectively spacing the pump out and into alignment.
Hard to say otherwise without looking at it in person.
