Like you, I'm using an Avon Roadrunner (4x19) at the back and am quite happy with it. As a front tyre I'm using the Metzeler Perfect ME 11 (3,25x19). It handles well and has the right look for an old bike, but it is not really cheap (around 66 Euros here in Austria). The Metz is a bit wider than the Avon Speedmaster. I think it was about 6 or 7 mm, maybe just 5, hmm can't really remember at the moment, but definitely wasn't much. Although both are listed as 3,25x19. Might be a problem with a very narrow mudguard, but it is okay for the valanced mudguard of the GF as well as for the sports type mudguard of the SS.
I found the Speedmaster to be a problem in wet conditions and having a rather short life. the Roadrunner performs better.