[Bomp]

Not sure about the spacing you're after. I've got the stock long bed for Daphne that'll need cuttin down. If you need some pre-cuttin measurements I can git them for ya.

Not tellin ya how to do your cuttin but I'd get as much metal down to bare metal for re-working (welding, hackin, poundin on).
When I did Steiners box I used a copper spoon for backing to help in filling those areas that had a bigger gap than desired. '66 box......not much help there for ya.

Glad to see you choppin on that box. Take your time on welding back together.

Keep on it! Slammin a beer to celebrate ya choppin.