Definitely self supporting but manufactering tolerances were not as we understand them these days. Is it the frame that is inaccurate from crash damage or original manufacture on old worn jigs, or are the parts you are wanting to bolt to it inaccurate on bolt centres. Engine crankcases are often inaccurate in fixing centres. As regards the undertray, then the sizes were fairly consistant and the bike would not have been allowed out if you had to spring the frame 0.100″. On that basis, I would think it a safer bet that the frame has had some bad experience and has been bent inwards.
It would be interesting to check if the gap between the lugs to accept the top rear engine mounting. From memory, it should be 7″. If the frame has been bent 0.100 at the rear of the undertray, I would expect that gap to be at least about 0.040″ undersize. If so, you either have to jack it out or reduce the crankcase top rear span. Scotts usually sort out if you have persistance! Kind Regards Roger