Measure 5 times cut once.
Measure 5 times tack weld — measure again and complete your weld. I had a professional do the welding — you do not want that weld to fail when you hit a bump in the road.
I did all the before measuring in my garage and then cut off the old one then took it to the shop — the guy was great and let me have the use of one of his bays while I did the set up.
I started with getting everything perfectly level positoned the new frame head and then carefully confirmed that all the measurements were exactly what they were with the old frame head.— front to back right and left and diagonals.
Hope this gives an idea of the process.
When the car was finished and I took it to the wheel alignment shop they checked the wheel base right and left and the tracking and everything was up to specs.
Phil