You have the strip speedometer judging by your other post – your 3500 looks very nice indeed, by the way. From memory when I had a couple of strip-speedo P6s you pull out the warning light strip (lift two spring clips above it to release), remove the windscreen pillar trims, then the padded facia top (two screws on its underside), pull the top forward and out, disconnect the speedometer cable and whatever wiring needs to be disconnected, undo two screws holding the instrument panel to the facia, and pull it out. I'm not sure how you separate the speedo from the rest of the instruments, but if it's possible it should be fairly obvious.