If it was just the rim that was offset that wouldn't put the chain alignment out.
I think with the shocks, axle and wheel bolted in it would be hard for the swingarm legs to get bent up or down (twisted) - whereas I know they can be bent sideways as I have one that is.
It is hard to know where to measure with these frames- notice how the right side footpeg has a longer stem than the left side one - the frame is not symmetrical, the lower frame rail is more inward than the left to make enough room for the brake pedal, kickstart and exhaust to all fit there. As such the swing arm isn't symmetrical either. Plus with a 40 year old frame who knows which bits may have gotten bent over the years...
I am afraid the only way that I can think of to know for sure is to try another swingarm.. maybe someone else in here knows another way - I am happy to take some measurements or more photos of mine if you like, John