I bought a very rusty 1980 D-100 for the 4 speed o/d and speed equipment it was equipped with. I drove it 30 miles home and it appeared to maintain 30 # of pressure at hot idle. With I remove the engine I found a 6 bolt pump bolted to a block with just 5 mounting holes. Is this common or is it best to replace the pump? Also what year was the block recast with the 5 bosses?

The 6 bolt is the older stile pump, the newer 5 bolt are the larger vol pumps with cast iron body, not aluminum.