Well, that Cologne V6 is a heavy old lump of iron, so uprated Dolomite springs would probably be a good idea, I think that driveshafts could be taken complete from the donor but shortened to suit their new location, similarly, rather than struggling for days trying to match the diffs front to rear and needing to have a bespoke propshaft of mixed origin, shortening the one from the donor and fitting its IRS to the Dolly-type shell shouldn't be too difficult to accomplish with care and measuring,
lots of that, in ascending levels of precision prior to cuts and joins being signed off.
A Ford (Essex admittedly) V6 has been fitted to at least one Dolomite. It doesn't have 4 wheel drive and obviously your proposed installation would need to see the engine sitting slightly higher to accommodate the transfer box from the Sierra, but as a source of inspiration, maybe worth a look?
Here's an underbonnet view of the car that I found on RetroRides just now:
Seems to fit nicely, the owner of that is a forum member here too, but I can't remember who he is.