Tire size for BSA A65 ?

I am looking at rear tires for my 70 BSA firebird and I would like to go a little wider does anyone know how wide I can go with the stock rim and swingarm configuration? I am looking at Avon super venoms. It currently has a 400/18. Thanks

 

I'm running a 120/80-18 Super Venom on my Ligthning without problems. Don't remember how tight it was. Let me know if you need to go wider and I can check.

 

Thanks I would be happy with a 120