ok heres a couple of things in relation to this (having just done 4speed to mt75 conversion)
Some mt 75 gearbox codes are
c8...ratio close box 3.89 or
e8 ratio close box 3.61 or
w8 wide ratio box 4.17
.......a lot of folks think the mt 75 boxes are all the same, but they are not
As already said your best bet is to find a whole donor vehicle to donate the parts you need .... these will be...
1) Gearbox !!
2) Rear Crossmember/Support
3) Transmition tunnel
4) part of the bulkhead
5) access to the chassis mounted bolts or adaptors as the front bolts on the crossmember wont fit.
6) Propshaft (one that bolts at both ends)
7) Rear diff ratios that are close to what you need, this will vary depending on your requirements
ok ... a quick guide (pls add extra bits too it as needed)
1) cut the transmition tunnel from your donor vehicle, going from the floor support between the seats underneath and all the way up the bulkhead as far as is practical, and extending half way across the floor (you can always trim it back later)
2) strip out your old engine/box/prop, then empty the interior
3) markup where your going to cut your floor and transmission tunnel, this will take some fiddling around, i cut mine i think somewhere around the heater box oval hole on the drivers side, use these kind of things to line it up and use your original floor line to give you the vertical position.
4) once tacked in refit your engine roughly to make sure it all lines up. then remove engine and seam weld the whole lot
5) Crossmember - a couple of options for this, you will need the correct one for the mt75 box though, after that the rear fixings will fit straight in, the front ones will not line up, so you can either just weld it straight to the chassis (not recommended) make an adaptor of some sort or modify the hanger arms, or do as i did and mark up new holes, drill them, and then cut the floor inside away above thse holes so you can fit new plates into the chassis and weld nuts to these plates fit them and then weld the plates inside the chassis. This can be done for the outside however, just make "c" plates with the nuts welded inside and then cut a bigger hole from underneath and slot the "c" section over the chassis leg, thus creating the captive nut you need to fastten it all up tightly
6) Propsaft, make sure you have one suitable for the job, dont try and use a petrol one on a diesel engine !, there are soooo many different types available we could be here all day with this, but a good guide is that a mk3 2.5di flatbed/tipper should fit a mkII long wheel base, also a mk3 minibus should fit ok too, dont worry about the length to the centre bearing when measuring, its not important, you can always move it !! the overall length is the important bit.
7) Rear Diff will be your choice in relation to what you want to do, just remember (as i forgot/found out) that the mkII axle is about 2 inches wider than the mk3 .... so a straight swap wont look too good. the diffs are interswappable, as for the rest i am not sure as brakes/bearings seem to be listed for upto 85 and 86 onwards
ancillary fittings i dont know enough about yet to post, such as speedo fitting clutch cable assy as i have not connected that lot yet and i will pprobably just produce stuff as i need it or source odd parts as and when
hope this was of some help to someone