Danny,
Is this installed in a '48 6400? Have you considered putting an original-style water pump on a relocation plate?
Do you know the condition of block's internal coolant passages? Crud often accumulates in the lower rear of the block. It is a bit of work, but with the water pump off (to install the relocation plate), you could try to loosen-up and spray-out crud by working through the drain-hole in the lower, rear, manifold side of the block. The crud and rinse-water would easily exit the block at the front of the block.
Tim
