Various versions of Type 9 boxes. Also various versions of bell housing.
Yes, you will need to modify the trans tunnel, possibly gearbox mountings.
The easiest option is to change the diff internals - JB Sports Engineering have alternative ratios. If you are doing the work yourself a 5 speed is nicer to have, though you will often find first on the low side, if a dealer/workshop are doing it you'll probably find altered diff ratios cheaper.
PM me your email address and I will email you copies of articles from various copies of Miscellany over the years where others have done this. Also have a look at MOCA web site.
4k rpm for 80mph sounds a little taller geared than mine - is that a true 80mph, or from the speedo?
Good luck! Oh, and welcome to the forum.
