Sounds like an awesome project, I was considering fitting a soarer turbo straight six in my old XM at one stage!
I think you should be right engine mount wise, does anyone know if the engine mounts on the 4.0i's are in the same position as a 4.1 x-flow? because I know the x-flows fit no worries. ( there is an XM ute with a t/charged 250 X-flow making over 340rwhp )
Castlemaine rodshop will be able to help with an adaptor bellhousing plate to fit a celica 5 speed or you could just go a T5.
Any engineering place should be able to make up a transmission mount for you. The diff is the old 2 pinion design, weak as piss, a 6 cylinder chrysler centura diff bolts directly in and you have a wide choice of ratios. You will need a better radiator, again the rodshop does a kit for you to bolt a v8 radiator in place, might work for an EB V8 radiator too, which will give you plenty of effective cooling.
Obviously the wiring is going to be the hardest part, you are better off buying a wrecked EF/L because you are going to need fuel pump, ECU harnesses etc. You will have to modify the fuel tank to add a fuel return line ( EFI ). you may even get away with running an "in_line" fuel pump instead of the EF/L original.
I would get in contact with Rod Hadfield as he may be able to help out with a lot of things
i think is the web address