I don't know how wide you can go, but I do know that if you lower the car, even a skinny 6.70-15 or 205/75/14 will be a nightmare to remove. Ask me how I know.
My 54 Customline has 235R15 . I have to use a bumper jack so the axle will drop down to take it off the car.You can not use a jack under the axle. Oh yea let the air out also. But my car sits pretty low.
Mine has lowering blocks so I have to jack the frame up so the axle drops, then kinda wedge the wheel underneath and jack the axle up behind it so it'll clear, it's a real pain. I hear some guys use air shocks and pump tons of pressure into them to spread the springs farther down when changing tires, seems like a good idea. I've also seen a few guys raise the wheel opening an inch or two which looks nice and probably helps clearance a ton. Unmodified, a 6" wheel is about all you're going to be able to run without serious issues. My 52 had 14x7" wheels on it when I got it but it's got a different rear axle in it (and they came off right away, looked goofy. 15" diameter is a must in my opinion)
My 52 Merc, all stock with original 15 inch wheels and P215-75x15 was all I could get in. Even by jacking up the frame and letting springs sag as low as possible it was still a tight fit to install. Plenty of room once it is in place though.. Not sure if a 225 would even be possible.
I have 235/75-15's on the back of my '53. It's lowered but not insane. To remove the rear tires the body/frame must be jacked all the way up, shocks have to be disconnected and drums removed. A bottle jack must make the axle goes as low as possible. Yeah, it's a bitch. Something I NEVER want to do on the side of the road.
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