As far as using the original placemant in the dash for the tach, all I can say is take it apart and see if you can make somthing work. Somthing custom would be really cool. Or perhaps somthing like the pic below. I did this with one of my old 5" auto meter tachs. I think it looks pretty good. And by the way. there is about 1/4" clearance between the tach and the window when the door is shut.
The hookup for the tach is on the ignition coil on the motor. There is a white wire that is not hooked up to anything coming off of the coil. It is about 4" long coming off of the coil and has a female spade terminal on it. If the truck didn't originaly have a tach, this wire may be taped up in black tape.