Just swapped out a 1992 305 for an early 70's 350 block. I have a few questions about identifying holes in block. Driver side, oil sender area ... see pic below.

1992 motor - the hole directly above filter (orange arrow) is for the oil sending unit. The hole marked #21 in pic seems to just go clear through into the flywheel area, and had nothing in it.

I didn't think to look on the 70's motor at the #21 hole, but there is a plug in the hole directly above the oil filter.

1) Does hole #21 in pic need to be plugged on the 70's motor?

2) Is there anywhere else I can attach the 1992 oil sending unit on the 70's motor, or do I have to remove the plug directly above the oil filter and put it there?

 

Yes and no Best to just transfer from your block to this block. But on the older (also newer ones too) blocks, Oil PSI was taken from the rear of the block, just behind the intake manifold...IIRC, a 1/8th NPT hole.

 

Hole 21 is not for oil pressure; it's for the clutch ball stud, in vehicles with clutch linkage.

The hole you have marked with orange is where the OPSU should go. I can't imagine how your sending unit would fail to fit. That hole has been there since 1955, as it's actually put there for a step in the mfg process (drilling an internal passage); its usage for the OPSU is incidental. For all the years up until they decided to move the OPSU to that spot, it just had a pipe plug installed in it.

Part # 11 in that pic is the other OPSU, installed in the location behind the dist.

 

OK, it should work to just leave the plug in the hole above the filter (hard to remove now that the engine is in car), and put the right combo of fittings into the hole behind the intake to attach my OPSU - right?

 

Yes, that will work; as long as the wires will reach. The 2 holes are functionally equivalent. Each of them is actually part of the block's internal passage drilling, and merely covers the end where the drill bit went in.