STUB FRAMING is simply cutting a frame piece off of one vehicle and welding it onto another.
Well maybe not simply, there are a lot of skills involved and an expert welder is absolutely necessary, this not a job for an apprentice so hire an expert if you don't weld for a living.
A smooth level concrete slab is necessary for proper alignment, your driveway just won't cut it.
Do NOT try to stub a unibody component to a frame, you need to construct a frame for the unibody first and stub that frame to the main, then you can fasten the unibody in place.
Choose your donor car and strip everything off the front end, likewise strip everything off the front end of the receiving vehicle, you need room to work and measure.
The most critical measurement is from the firewall or a stationary cross mount to the centerline of the front wheels on the receiving car. The wheels need to fit in the center of the fender well openings so front to back alignment is very critical. The ball joint grease fittings or centerline make great reference points.
Put the receiving car on four blocks, the same thickness, under the wheels so it is sitting with the springs loaded or as you want the finished car to sit. Measure from the frame to the ground at four points and block the car by the frame so the measurement is the same as the original removing the blocks from under the wheels. You will be using the blocks from under the tires to line up the stub frame to the proper height.
Remove the drive train from the receiving car and compare the frame widths near the areas to be cut on both cars to find a good balance in overlaps from side to side. Use a square and cut the receiving car's frame square with the floor and a half inch longer than required. Put a level to the donor car's frame in an accessible area forward of the cut line and mark a line to level the frame at installation. Cut the donor car's frame square with the floor leaving it a half inch longer also. That leaves an inch to fudge with.
Roll the donor frame onto a set of the original wheel blocks. Yes, it is tough because the drive train is still attached. The reason for that is to make sure it will tuck in and to preload the springs. If the donor car is significantly lighter that the receiving car you should pile on weights to approximate the additional load of the receiving cars front end.
Position the new frame so the frame ends butt each other support the new frame ends so the level lines are level. Check for drive train alignment and you now have an idea of how the pieces will be mating. Check the wheel center measurements and you should have an extra inch on both sides.
Next you need to laser align the two frame pieces so the car will track correctly. Use a $5 laser level from Harbor Freight or even one of those toy laser pens. Lay it on the frame at several points and be sure the frame is centered left to right and not cocked at an angle. Time consuming, you bet but nothing says stupid more than a car going down the road at an angle so don't cut corners on aligning the frames. When you satisfied it is ready to weld, cut the half inch off the receiving frame. You can then weld a couple blocks on the bottom of it to support the donor frame at its proper height while you grind the donor frame to fit flush at all contact points.
One more complete measurement of all angles and the centerline distance. Have another person check all the alignment also because by now you are getting anxious. When all is OK tack it and check it all once more. No need to go into spacing plates and overlap plates, a good welder knows about all of them, so just cut them as he asks for them.
Here are some scans from FYNLRYD'S Stub Framing, sorry about the quality but they were not taken for the web and were a bit dark. Click the pic for a closer look.
So what is next? Remove the drive train, roll the chassis outside and sandblast all the frame work. Now is a great time to replace all the bushings, ball joints and tie rod ends. Blast and coat everything and you are ready for years of safe enjoyment without costly repairs.
Fit the front end back on the new frame and fabricate new support brackets as needed. Fit the steering column from the donor car to update your original one. Fit the new drive train and make new cross members if needed then make a new driveshaft out of the old ones.