V8 is easy, you just need to adjust engine mounts.
As for the 5 speed box, I looked into this earlier this year and it it a lot of work (and probably not worth it).
Front axle needs to be changed for one with CV joints (costs money)
Both Prop shafts will need to be custom made (costs more money)
gearbox mounts need to be bought (costs money if you can find them)
LT77 VERY heavy box to put in from the inside, need to replace the crossmember with a removable one (costs money)
Modify the cross member below back of engine (pain up the ...)
Reconditioned Series 3 box about £300 (much cheaper than all above and a lot less hassle, can be fitted in a few hours)