Montego is front-wheel drive. simple answer is no (without major mods) Gearbox is (IIRC) Volkswagen 084/085 (or similar code) with internal diff. D/s flanges 6-bolt and nearly in line with 'rear' of engine (backplate) so loooong rear p/s, v.short front one. Do you know what 'box is in at the moment? If it's a series, may be worth seeing if purchaser will just take engine. If it's a RR/Disco 'box (permanent 4wd) I don't know about fitting Prima to it at all (don't even know of anyone who's done it).
Techie bit: if you're unsure which box, series has high/low (forward backward) lever with red (or yellow) knob on top and 4wd (pushy downy) lever with spring on and yellow (or red!) knob on top. RR boxes either have air difflock (centrediff) usually mounted near base of gearlever looking a bit like a cigarette lighter and a high/low ratio lever beside main gearlever (usually black knob that looks like an oversize Swizzels lollypop) and diff lever where handbrake usually goes or a (almost) 'H' gate lever for both functions beside main gear lever. Think that about covers it. Think propshafts need to be changed too (I know for certain that when I swapped 4spd auto with chain/gear transfer box to 4spd auto with viscous coupling transfer box propshafts had to be changed - so if series propshafts match one or t'other, then one has wrong length 'shafts (Think that makes sense!) Free-wheeling hubs and and overdrive worth finding for extra mpg, RR diffs too. Even tyre choice affects mpg so if it's all roadwork, narrower,taller tyre helps (as obviously does 7.50x16 rather than 6.00x16 - larger rolling radius). Don't know if London taxi (black cab) tyres are easily available but I seem to remember they're 16", tough as nails and about as grippy as icy teflon - but may be worth seeing if 'cab' breakers have any. Ex-military (XCL?) tyres ok onroad and gentle-ish - ok for minor off-roading too.
Of course, this may make your speedo inaccurate but:
A - this isn't really that much of a problem in most Landys.
B - No different from how 'accurate' speedo was before
I'm still looking for lettering (like letraset?) for sticking on dial to match numerals so mine can read things like "60-ish,70-something,roundabout-90 etc"...
Tippex dot/stripes on glass marking 30,40,60,70 (get a friend to 'pace' you) cures this
HTH