In preparing to install the oil pressure tube from engine to gauge, I noticed it has an "L" shaped bracket that is supposed to mount on the left side of the block with a 1/4 x 20 screw that is already screwed into the block. In looking at my spare engine with its oil pan removed, it appears that screw holds an oil line in place on the inside of the engine. If I take out that screw, are the odds good that I'll be able to screw it back in without problems?

You are looking at the wrong screw! The "L" shaped bracket for the oil pressure gauge line attaches to the top of the flange of the block via one of the oil pan bolts that holds the oil pan to the block. The bracket does not attach to the screw on the block that holds the oil line in place on the inside of the block.

A long oil pan bolt screws all of the way through the flange on the block from the bottom and the "L" shaped bracket attaches to the long oil pan bolt on the top side of the block flange.

Thanks Dog. A follw up question-
"the "L" shaped bracket attaches to the long oil pan bolt on the top side of the block flange."

None of my oil pan bolts stick up above the flange. So, I'll need to replace one with a longer one. Is it the bolt just behind the exhaust pipe? Is the "L" bracket held in place with a square nut?

As I remember the bolt is in line with the exhaust pipe and a square nut and a lockwasher were used to secure the "L" bracket.

Thanks Dog. I'll make the changes.