I have wasted much time and energy hooking up the oil pressure gauge on my ’45 Chevy 1/2 Ton. I can save you all that trouble here. 

The main challenge is the connection of the 1/8” oil tubing from the engine block to the back of the oil pressure gauge in the dash. It requires a “special” fitting that is best to replace with a new one to avoid leaks. 

The "special" brass fitting is best known as Weatherhead/Eaton 6100X2. It’s described as a “Threaded Sleeve Nut.” Don’t waste your time looking for one at an auto parts store or online unless you want to buy 50 of them. Go to your local hydraulic supply store. Mine had 120 of them in stock.

My next order of business was the 1/8” tubing. My original equipment was mild steel tubing. Good luck finding that anywhere! However, Chevy using steel tips you off; it should be tough. Don’t buy one of those copper oil line kits. They are very thin-walled copper tubing. Buy your 5’ piece from that same hydraulic supply store. It’s about 2X thicker-walled than the kits. 

If you want to stick with the original way Chevy connected the tubing to the engine block, buy a second Weatherhead/Eaton 6100X2. Also, get an adapter for that threaded sleeve nut to the 1/8” NPT outlet on the block. On a 235, the oil pressure outlet is on the left side of the block, about 1" from the back and about 4" up from the oil pan lip. 

So, that’s a total of four (4) parts from the hydraulic supply store—one stop.

In your routing of the copper tubing, there is a significant vertical section from the block's outlet to the grommet in the firewall. The wear on my firewall grommet tells me a lot of engine movement is transferred up that vertical section of the tubing. Chevy put a horizontal loop in the oil line after it came out of the engine block to dampen all that movement.

There you have it. Comments welcome.