So, from what others have said, am I right in thinking that it’s the length of the arms that is a little restrictive, rather than the lateral spacing between them? In other words, the diameter of the wider tyre may catch the cross tube…..or is it both dimensions?
To lengthen the swing arms would be fairly straightforward. Cut the arms on a straight part, make up a stepped spacer plug with a suitable length, weld back together, file smooth, paint, fit a slightly longer chain.
I suppose the width of the assembly could be increased in a similar way. But yes, it would be vital to keep everything in line. My old BSA C15 had a very strange “stance” in that when the rear axle nut was tightened right up, the wheel leaned over to the left. I think it was shoddy quality control at the factory because the hub plates weren’t parallel. But then the whole bike was made to a fairly shoddy standard (although as an inexperienced teenager I thought at the time it was the bees knees).