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.
You do not have to remove the fascia top or the A pillar trim if you don't want to. That said, if the facia top has dried out and gotten brittle, the lower part of it may have swelled making it hard to get the speedo head out.
You only need remove the light strip and cover and then reach up behind from inside the glovebox and disconnect the speedo. There is no need to remove anything else as the unit will pull clear enough to disconnect any cables. In fact you cannot remove the dash top first as there is a fastener hidden behind.
Disconnecting the speedo and reconnecting it isn't easy and I have smallish hands.
I swapped from the strip speedo to the round dial dash, so I should know!