I have a very nice B-body oil pressure gauge in a '68/9 instrument cluster that is in the position where you usually see a blank, clock or the tach. I have never seen another one like it. I'll have to dig it out and take a pic. I'm thinking it'll end up in my RR.
I had a ralleye dash in my 69 Bee and it had an oil pressure gauge. It was a round gauge. Are you talking about a rectangular oil pressure gauge for your RR?