Agree with what Rog G says about the propshaft. I have done this job with the bevel box in situ and out of the car, I think it is easier in the car fitted as it is anchored down properly, although a little tighter for access. It's a lot of unnecessary work to remove the bevel box just to replace the seal. If you cut both of the metal brackets the propshaft can be removed to give you good access to the pinion nut, no need to disturb the rear wheel or drive belt. Just drill the brackets either side of the cuts and bolt a joining plate over the bracket cuts. It will be there for the next time the propshaft needs removing and just as strong to retain the propshaft in the event of failure.
The Viton seal you have the link for is the correct one.
Don't forget that the pinion nut is a LH thread!
