Rear tire size needed for 340 stroker?

you need to figure out what torsion bars, springs, and shocks you have.

are you concerned with cornering traction more than straight line???

correctly spaced wheels and maybe offset spring hangers will get you 275 rears, which will provide great street traction with a good tire. you will also need to increase your front tire size for handling and braking during increased speeds. spring rates and shock design will affect traction significantly but differently during different types of driving.

There are more educated gurus on this forum than me, but more tire means more traction, and with around 500rwhp it is hard to have too wide of a tire.