New guy with a question

Hi I'm new here and thought I should introduce myself and pick your brains while I am at it. I have a 70 BSA firebird with clubman bars homemade rear sets. Motor is bored .040 over but otherwise stock. My question is I want to put a new rear tire on and would like to go wider. I was thinking along the lines of a Avon super venom. How wide can I go with the stock swingarm, rim and chain configuration? Any help would be apreciated.

Terrence