Hi Saul; - welcome to the FHRV forums. I am in a similar stage as you seem to be. I am presently working on a frame-off restoration of my 1973 Boler
; removed shell, had existing frame sandblasted, painted, and new axle
(with brakes) installed. My frame is solid, not bent or twisted, etc., and therefore didn't need any further work. The shell dropped back in with no sizable gaps. If you still have the previously 'bent' frame, I'd be checking the accuracy of your newly, rebuilt, replacement frame. I do realize there is a small amount of 'play' between the shell and the frame; but, I believe my shell is adequately supported at all contact points. As suggested by Paul Nieumeister (the Ontario 'Guru' for any Boler
repairs), I did have a welder ADD four angle-iron tabs to my frame; near each of the four corner contacts; and have now installed four 3/8 by 1 1/2 SS bolts, and fender washers, for additional security. I wasn't impressed with previously having only 16 'screws' holding the FG shell to the frame. Good luck with your trailer body-frame allignment.