xs 250 16 inch rear wheel

Hello everybody, Not been on here for a while, I am currently building a 1978 xs 250 from scratch into a bobber. I have gone for an 18 inch front wheel and a 16 inch rear.

The rear wheel came from a 250 custom model the rear tyre size is 120/90-16 ( standard size ) I am looking for the largest tyre I can get on the rear for that fat look, Does anyone know or has anyone experimented with this size tyre ??

I currently have an  140/90B-16 AVON AV72 77H R RF  COBRA  on the rear, The wheel will fit on only it is ( standing at the rear of the machine) very close to the swing arm on the R.H.S. and very close to the chain on the L.H.S. 

Has any one had any experience with this?? I have maybe been thinking of a 130/90-16 ??

Can anyone advise