Since you have a donor truck, you have the answers to the hole location problems right there.
You also have every part you need to do it.
It's really as simple as transferring the front spring brackets from the 4wd to the 2wd frame, as these are probably the most universally located holes on the front of the frame, AND a very good starting point to using the springs themselves as a start to locating the rear mounting points.
It has been said that ONE pair of the 4 pairs of bolt holes for the engine crossmember is located where the crossmember needs to be.
Technically, you could "guess and by golly" it from there on the engine crossmember.
Locating the trans crossmember on a 4wd is really a matter of bolting the crossmember to the transmission, jacking it up (engine all secured properly), centering it equidistant side to side, marking and drilling the bolt holes.
The UPPER 4x4 trans supports are slightly more troublesome, but can be laid out, and drilled from underneath, but, if you are a purist, you'll need to jack the cab up to put the bolts in from the top, as stock does.
FYI, if you remove the engine and front crossmember, it's possible to slip the engine crossmember into the frame by flexing both framerails outward slightly to allow clearance for the 4x4 engine crossmember to work into place.
I have an '85 CrewCab that I have been contemplating this conversion on, but, IMO, a 4wd longbed Crew is not a practical mud/snow rig and I like being practical.
Mark.