AMC 401: Questions about 2 sets of holes in the block

I am finally assembling the AMC 401 to replace the 304 in my 1978 CJ5, and I have two questions related to holes.

(1). I see a hole on the passenger side of the block that I neither know what is for nor what to plug it with. Please see the attached picture. The hole is on the passenger side of the block, opposite from the dip stick hole (which is on the drivers side). There is a also a bulge in the oil pan at this location, maybe it is for a sensor or something? What is this hole for? Is it threaded? Do I just put a bolt in it to seal it? Do you know the thread size and pitch?

(2). I see two holes in the block behind where the oil filter is located. Looks like one of the holes is 1/8'' NPT and the other is 3/8'' NPT, but this is just a guess since I believe they are for the oil pressure gauge. If I look at the 304 in my CJ5, it looks like the oil pressure sender for the stock gauge is attached to an elbow that is threaded into the 3/8'' NPT hole in the block. I can not see under the 304 (due to location, headers, oil filter and grime) to verify that the 304 has both the 1/8" and 3/8" holes (I was hoping that the 304 would also have the 1/8" hole but have a plug in it). Is it typical for the oil pressure gauge to be in the 3/8" hole (actually the oil sender is attached to an adapter that transitions from 1/8'' NPT to the 3/8" NPT into the block). Why are there both the 1/8" and 3/8" holes in the block? Does it matter which one I use for the oil pressure gauge? I am planning to also connect a mechanical oil pressure gauge to monitor pressure during break-in.

If anyone can shed some light on these holes, I would be most grateful.

Richard

1. Oil pan rail bolt hole and boss for clearing the oil pickup tube. 2. one tapped hole is for the oil pressure sending unit, the other is for the draining the block of coolant.