Firstly, you need the right type 9 box! If it's from a V6 the first motion shaft is too long.
Although usable, it requires a spacer between the box and bell housing to take out the extra length, but that puts the gear stick further back, and means you need a shorter prop shaft than that which is readily available.
Birmingham Minor Centre (now David Manners Group) sells the kit to fit one - that's where I got mine.
Did the whole conversion in a week (evenings and weekend), but did a fair bit of prep in advance so that the car was off the road for as little time as possible. I built up the box on the bench with the bell housing and clutch adaptations. And modified a spare handbrake lever so I could just swap them over when the time comes.
It's not a cheap conversion, but if you use the car daily, especially on motorways, it's great.
I tried the 3.9 and couldn't cope with the reduced acceleration.
Five speed and standard diff means all the normality of standard minor gearing (plus synchro on 1st ;) ) and the benefit of the longer gearing of 5th. Means you don't have to trade acceleration for speed.