I made 3 temporary crossmembers exactly the right width. They bolt in and can be moved when working out how to fit everything.
The two chassis rails. I was going to have them angled so it was thinner at the front than at the back but decided against it - I don;t think I'd get everything lined up. Eventually decided the chassis rails would be 26" outside to outside.
Another pic of chassis just before I cut the end crossmember out. It's too wide.
After sitting outside for 10 years- this is the Austin Morris Sherpa chassis. I welded some 30x30x5mm angle to the inside. One length was drilled every 40mm with a 10mm hole to make it easy to trail mount.