If its an S not an auto then I would advise that you remove the existing gearbox mounting brackets from the trans tunnel. It really is quite a tight fit and every little helps. this will also let you use the drive for the speedo by cutting a hole for the cable to com through.
I got hold of a SD1 speedo cable and took the gearbox end of it off and put it on the p6 cable, there is enough length if you re route it inder the carpet you also need the drive gear from a SD1 with a mechanical not a electronic speedo.
Dont forget that you will need an SD1 spigot bush that goes in the end of the crenck shaft (it will work with the p6 one but the gearbox bearings wont last long. and ther only a few quid from rimmer bros.)
clutch slave cylinders are the same (p6 or sd1)
the clutch part you need to change is the friction plate. (differnt splines) you can re use the p6 cover plate.
P6 auto propshaft
Give yourself plenty of time to sort out a nice gearbox mount. I didnt and it fouls on the exhaust! it should be possible to make it the exhaust fit, but you have to muck about with the front mounting bracket that used to bolt to the gearbox.
Good luck, it makes the car a lovely cruiser and much more useable if reverse and second didnt work on your old gearbox!