the head bolt threads in the earlier blocks are basically flush w/ the deck, the later model block have the threads recessed below the deck about 9/16 of an inch( which is better b/c when its torqued is doesn't distort the deck as bad), ran into this when building the older block in my 84 vs. a 109 block. and therefore require a slightly different length, its probably not as big a deal w/ bolts since they got into water, but w/ studs you'll have the shank of the stud going out past the head.
