ramcram

Can anyone tell me the difference in the engine oil level sensors 78-87 and 88-95 please?
I'm changing my '86 trans oil level/filler bottle so it can be filled and 'dip sticked' through the rear floor and I'd like to incorporate a low level warning light.
I'm considering a cylinder [short] behind the rh axle, between the fuel tank and the rh coil spring. this will sit on [welded to] a rectangular tank at the height of the original one and connect by hose through the 'up' curve in the rear cross member, to the trans sump pipe.
also, this sump pipe has a thread inside it. Can anyone tell me what the thread is? the pipe OD is 18mm.