You're lucky in that front driveshafts, hubs and drive flanges should be adaptable from a 1500 (FWD, but the Dolomite subframe has the same webs with large slots for the driveshafts to exit) so it'll not be as difficult as trying to fit the extra pair of driven wheels to something RWD and with struts up front. Maybe look at using the Ford's pot joints instead of the Triumph's rubber doughnuts though but.
It'll be
like a 1300 4wd (Pony?), but with more engine. Given that you're going to be faced with making the odd cut here and there anyway, it hardly matters whether you're cutting down by an inch or a foot as long as parts that have been shortened are welded back together well, and you have what it takes to do that.
Sierra uses a semi-trailing rear that has remarkably similar geometry to the 1300's IRS at a quick glance and the Dolly's live rear end picks up at the same points on the shell, so, with some 1300 suspension bits and a means of attaching the rear diff to the floor of the Dolly shell at the right height, you'd be half way there. If you could pick up an ex-Middlebridge (or very late Reliant) Scimitar 2.9 instead of the original Cologne with its siamesed exhaust ports, then you'd have unleaded heads, greater efficiency and an even better noise.
When I was with IM and the layout sketches for the then-new Legacy came over to the UK for the first time, we fitted its single belt flat 6 engine, its uprated gearbox and its (cut down) drive shafts from both ends to an older L series shell that we had in central stores in order to try out the all-new oily bits in a car that wouldn't attract attention. Yes, careful measuring and some jig work was undertaken, but the first step was to lay out the new drivetrain components on the floor along by the L series body and break out the yellow chalk.
Doing the same with every component of your XR4x4 would be a good plan.
Thread spotchered, I'll be enjoying this one.