Does anyone have a picture of your oil pressure gauge wiring by the engine? An actual picture. I looked at the schematics and purchased the Dodge repair manual, no help. It looks like you leave the engine oil light sending unit in place and tee off to the mechanical sending unit. Any thoughts would be helpful.

PS: I've posted here about my engine running poorly after the carb. replacement. It turned out to be the plugs. Yes, I know HEI is the way too go, but stock is were I'd like to stay.

1983 D150 Slant Six 81500 Miles.

Your assumption is correct, the factory uses(depending on application)
either a "Y" or "T" adapter, and the wire for the gauge may or may not be
taped to the engine harness somewhere in that vicinity. Ma Mopar used
to have the wires for optional equip. part of every harness for a particular
model, My '72 /6 swinger had A/C, but a foot pump washer setup. The
killer was the wiper sw. was even marked "wash", & the brown wire was
run to the firewall. I simply added a reservoir w/a pump and a wire to the
firewall terminal and SHAZAAM!!, power washers. So the wires may
be there already, see what You can find............