The 318 poly had a forged crank. Bearing size is the same as the 340. However, the 318 poly is balanced for light duty rods, not the HD pieces found in the 340.
Most of the posts on this page are correct. I use the 318 poly cranks when I can get them.
You should ALWAYS balance. Never ASSume on that. The flange is the same for flywheels. Can't say anything on the TQ register. If it is different, any local converter shop can do a replacement converter for cheap.
