"...The replacement engine does need to be gone through..."
Need to make a positive ID on the block & heads.
(1) What is the casting number of the block ?
(2) What heads are on it ? That's the number cast over the center exhaust ports.
The most common 400 block around nowadays is the "557" block. If the block you mentioned has a casting number which ends with 557, I wouldn't spend a penny rebuilding it. Those blocks have weak main webs which have resulted in many web failures. Some brag about how much power they have made with these blocks, without any problems. But, others had had failures in near stock rebuilds. No reason to take a chance IMO.
Most agree that the 481988 block is the best, commonly available, all around choice for a stock block engine build. They have the thicker main webs, and have 5 motor mount bolt holes, so that they can be used in most any Pontiac chassis. They were used in '71 thru early '75 models.
