As i looked at the wheel, the rust revealed that it is a 4 3/4" gm bolt pattern, those are dual bolt pattern wheels. No gm came from the factory using that size
There is a local triumph club so there are quite a few cars. the radial is as you said, the tr6 went to 185/70. I bought a set of michelins 225/50zr15 but never mounted them as they are too wide.
I saw the tires were cracked, and just show material but some of the older show cars like to run with bias tires. Radials were factory options since the early 60s