'81 RRC retrofit questions

I'm working on an '81 2-door that's had the 3.9 (complete with EFI) swapped in out of a '95. Some of the wiring never got finished so I'm trying to sort it and have a couple of questions regarding the engine oil temp gauge. These early trucks aren't common over here so this is the first time I've worked on one this old.

Mainly what I need to know is: where was the oil temp sending unit originally located? And what ohms resistance is it? Can I use a water temp sending unit in it's place, are they in the same range?

yes..just use it and see what happens.

Thanks. I still would like to know where it was originally installed...in the sump, or in one of the cooler lines, or did it thread into the block or front cover somewhere?