A 120 is going to have some rub. I'd go the 110 Venom route just so you wont have to fight it in there. Also make sure your fender is in fact a 1970. The earlier models, I think 1966 and earlier, had the tall skinny 19 wheel. The fender matched, I had to run terribly narrow tyre the first year I had a BSA.
Aaron
Aaron