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Hmmm, not sure what block you have.... My 400 block had a threaded hole the right size on the passenger side into the water jacket-just like the original block.
I am using an early 70's casting. There is a coolant drain plug on each side. The factory setup has the single knock sensor on the passengers side.
Note: There is also a knock sensor shield that really amounts to a piece of stamped steel, kinda at a 35 degree angle...possibly to keep exhaust noine from triggering it.
If this is for your 91, don't forget the earlier blocks have a passenger side dipstick tube. You will not be able to use your factory pan OR factory pickup tube.