Might be best to just start mocking things up. I don't have any step by step instuctions, but will offer a bit of advice for you.

1. Before you make any cuts, make sure the frame is supported very well, some type of fixturing to prevent the rails from moving around when you cut them.

2. Mock up your suspension and rails at the ride height you want them at, make sure this is the look you want.

3. Measure twice, cut once. Once you have your frame secured to some type of fixture, make sure your cuts are where you want them, and are the same dimension on either rail.

4. Just tack the Z and the rails in place, measure everything from front to back, side to side, and corner to corner a few more times to make sure everything is square.

5. Mock eveything up again, suspension, body, motor & trans, and steering. Make sure it looks the way you want it to and everything fits.

6. Don't weld the whole joint at once. ie weld the top side of the right rail, then the bottom side of the left rail, skip around on your welds to keep from twisting the frame with the heat from welding. Again I would stress how important it is to have everything secured to a fixture when doing this.

This is by no means a complete instruction, just some hints to make the job go better, and the frame to be square when you are done.