What is the biggest rubber I can run on this body?

This isn't actually my car, But the owner lets me race it sometimes. It has a grunty wee 340 in it, that came out of a class winning marathon boat.
Unsure of the cam specs, but it does run real high comp domed top forged pistons etc.




anyhow, whats the biggest rear tyres you guys have managed to fit under the guards on one of these? It's aussie assembled, but I think its similar to a '67-'69 dart?
The owner wouldn't dream of even so much as lipping the the gaurds to accomodate wider rubber. Last time I raced it (running the 14x7 mags and cheap nasty 215/60/14s off one of my cars) I could barely touch the throttle out of the hole without smoking the rears up badly.
So what width tyres have you guys managed to fit in there?