Looking a little tyred

The rear wheels looked a little tyre'd...R.D.R.R. I couldn't imaging driving on the original 6.75 x15's, 3 ply, nylon tyres. (I will sell the one shown here in pic. if you are interested, just send message).
I have measured the rear wheel arch so many time that in the end all the numbers were mixing in my head. So, Q: what is the widest tyre that I can fit under a stock 53 Cuzzo wheel arch????
A: 255x60x15,
11" at widest point, 27" dia. I can place my hand between the tyre on each side and the arch.
At the moment I am having to drop the front spring shackle to get the diff low enough to remove the tyre.
I'm getting good at it now Removal is down to 10 min. That's in a garage with all the tools at hand. The usual test will be on an un-even embankment in the middle of nowhere in the pouring rain.
I have other options, but I'll save them for next time...