biggest tire for my Valliant

You can find hi-po 15’s but they are very low profile and too short for an a body unless you are doing auto-x.

https://www.tirerack.com/tires/toyo-proxes-r888r
Mopar Action's Green Brick ran the same 225/50VR15's My '88 ShelbyZ came with when it placed 3rd in the One Lap of America, and it's the size I have on My '64 GT on Popo Rallyes at the moment, they are low & some may not like the look.....but a little free gearing bump if You have a dog rear gear.:D...