Frame rails

Its not that hard, measure from the front crossmember side to side and note where the spot welds are, clean up the old rail where the spot welds are, then drill out the welds with a drill or use a special bit for cutting out spot welds, take your time and try not to go through the floor, if you do its not that bad you just fill them when you weld. draw a line on both sides of the rail lips on the flloor so it will be easy to put in the new one, then remove the old rail. if you are just putting a piece in then just but them together. If you dont have a spot welder the plug weld it in, holds better and looks original.
Take as much weight off the car as possible, like for the front rails the engine and trany should be out. put stands underthe car so it dont fall when you take out the old rail. measure what you can and right them down, Its also good to take digital pics of everything ifs its your first time, the more pics the better, you might not get it done for a few days and you cant remember everything
Note: not all the chassis are what the book says it should be, thats impossible, so if your going by the book dont be to critical.

I do have the specs but my new Brother MFC printer combo took a shi* already.