Hi Mark,
Eric said - Yup. Lots of guys are doing that. Greg (Austin Vert) might have the best example, since he's rolling a vert as well.
Make sure you get the backspacing right, otherwise they'll stick out too far like mine do.
Eric is spot on there with good advice. Also, check out what Mezapu did with his 10x15 Magnum 500's at the back. He went for a bigger backspace allowing the rim to sit in a lot further, but he had to do some modding to the tubs i think.
OK - In my case, i've got the '73 Mach 1 and the '73 Vert. On the Vert, i'm running 10x15 Magnums with a 5 inch backspace. I have not touched or modified my bodywork at all. However, on very rare occasions i will get a very slight small rub effect with the tire on the outer wheel arch lip if i go over a large dip in the road, and with a full tank of petrol. This does not damage or deface the wall of the tire in any way. But that said, you will need to install good quality five leaf springs as well, if you have not already, to give the rear end the support it needs to prevent any serious rubbing issues. I also run standard eye shackels as well.
On the Mach, i bought US Indy mags 9x15 with a 4.5 backspace. Five leaf springs are installed here as well. I get zero rub issues with this setup.On both cars i run Mastercraft Avenger tires 295x50/15 to go with the rims.
That's my story, and i'm very happy with both setups.See my pics below.
Greg.