Big cam will say big cam on the side of the block. They made a I,II,III,and IV. The I used short head bolts that cracked the deck of the block II and III were both good IV used a special radiator for the aftercooler. They started making the BCI in 77, the II in about 80 and the III in 83 and the IV in 87 and then of course came the N14 which was available for a few years in Mechanical or electronic injection and then electronic only from about 93. As for the tach question, I think mechanical tachs are pretty much standard. They drive off the fuel pump with a cable like a speedo. If a cable is really dry or the head is rusty it will show grossly false readings. I don't think your problem was from over speeding. These engines were meant to operate between 18 and 21. Allowing an engine to lug at 1600 would easily do the damage you describe.
