Rear tire size needed for 340 stroker?

No. If that's the route you want to go, you need the whole kit with the offset front hangers, and new spring perches for the rearend housing as well. Or you can just drill four new holes for the front hanger inward 1/2" and reuse the stock front hanger, do something similar in the back and just buy new spring perches. Do you have welding capabilities? Because, you'll need to in order to move the perches on the rearend housing inward.