Referencing the front frame is especially important because that is perhaps the most critical relationship if you want the car to track straight down the road. If the new frame is straight, then all these measurements will just confirm that you have everything in right. | 
If your new frame is distorted in any way, these measurements will show you where and by how much. |
Now it's time for the boring part. You'll need to find and drill out all the spot welds that are holding the old frame in the car-and there are a lot of them. | 
A 3/8-inch Blair plug-cutter drill bit works great. Referencing the donor frame will help you locate them all. |
Even after you drill all the spot welds, you may need to run a chisel under each of the spot welds. That way the frame will separate cleanly from the rest of the car. | 
Your donor frame will likely not come ready to install. This one-from a four-door car-came from a salvage yard with the rest of the back half of the car still attached. You'll need to remove everything you don't need. Since our car needs a new rear panel and this one is good, we decided to install them as a unit. |
Don't be surprised if your donor trunk floor is bad. just install it like it is and plan on replacing it later. Leaving it in place for now helps to keep everything square and rigid. | 
Here's where it helps to have an extra pair of hands. You'll need to lift the old frame section out of the car. |
There sure isn't much left, but this really shows the importance of lots of support for the rest of the shell. Because nothing moves, every part will fit like new. | 
Here you can see we had some rust to deal with that wasn't part of the repair we're working on. Might as well fix it now. You can see the repair made to the top of the lip and the side rust is just about fixed. |
With the old frame out of the way, hammer and dolly all the mating surfaces of the remaining material flat. It's difficult to do this with the donor frame in place, and it will make welding the new frame in place much easier. | 
Even with a body that has been completely stripped, there will still be rust left over inside the factory seam locations. Clean off every seam, and then spray 3M's Weld Through Primer on all the mating surfaces. Also, it's a good idea to paint the inside of your new frame with a rust neutralizer such as POR 15. |