Depends on where you're going to mount the shoulder strap anchors. If you mount them in the roof rails as I did on my son's '65, you want to do the belts first, as I wanted to do the welding in the itnterior with no headliner or carpet in the car.
I've seen another VMFer that used the quarter piller for the extra mount point. He did a very nice job, and that way could be done at any time, as there's no welding required.