340 Rods, speed pro L2316F “stock” weight is more than stock piston

There are balance pads on the top and bottom of a factory rod. These pads are routinely ground down when balancing a motor, and no, doing so doesn't weaken the rod. Based on my limited experience, factory rods will vary in weight a lot more than aftermarket rods, which typically don't have those balance pads.

Heed the advice above. Don't grind on the rods yourself. Balancing is more than just making all the rods and all the pistons weigh the same, although that is part of it.