You imply from noting the concern about mpgs that these conversions will be for daily drivers, is this true? If so, frame and body changes usually have to be kept to a minimum, adding roll cages, stiffening, etc., to cope with the added stresses yet still remain street-legal.
If not, then it depends on what drive layout you want to be. FWD (cheapest), RWD (pricier but more options), and AWD (not really an option unless you have some SERIOUS horsepower you want to put down and have the $$$ to back it).
You'd have to look carefully to see if an manufacturer's engine type was used in a FWD as well as RWD configuration to find a tranny that would work...GM's 3800 series V6 is a good example.
The second easiest (hah) would be to modify the FWD to accept the RWD drivetrain. Major issues would be engine/tranny mounts and subframes, firewall, driveline "hump" for the driveshaft, roll cage, and rear suspension anchors.
I'm sure there are others way more into this than I, thus would have much better insight, but I hope this is a decent launch point for discussion.
Hope this helps!