Mechanical oil pressure gage help.

I ordered a new sending unit, mine has a crack in it (not leaking) but when I start it, the light goes out then comes back on in 2 to 3 seconds, It did this back this spring and I cleand the wire going to it and it worked correctly , and stayed off.
Know it just goes out for 2 to 3 seconds and then stays on..
I changed the oil today and was going to change it out, but as you see it is the wrong sending unit for my 66 model 170/6, my sending unit looks like it has never been changed and has 2 times bigger threads on it. 68.000 mile on her.
Is this where I need to put a mechanical oil pressure gage or is there another
location to install a gage at..

Thank you for the info, I hope everyone is having a great evening :D