Rods are the same for all 340's, stock pistons weigh the same; the difference is the compression height between the cast/forged crank 340's. B&M makes a flex plate with the correct scallop for a cast crank motor, or most converters can be ordered with the correct weight.

Crank swap isn't really necessary unless you are pushing it really hard. The cast 340 cranks are pretty stout.