1965 frame back half help?!

[Dwolfe]

Ok guys so i have decided to back half my truck, im going to lay frame , so the vertical distance from the bottom of the 2x4 steel tube to the axle will be 10 inches. No i have marked were im going to start my back half but im trying to be as perfect as possible and i dont know exactly what angles to cut the tubing were the diagnol meets the straight half of tubing. I know it depends on what angle i put the beginning of the frame at but any ideas on which angle will
The tubes line up even? Or will they not? Thanks!