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

I Agree....get that reciprocating assembly balanced.!! :) You'll need to take the following to the machine shop. Your 8 pistons, 8 rods, crank dampener, flywheel or flex-plate, one set of rings, one set of main and rod bearings. Let the skilled machinists handle the rest. Locally by me, Holbrook Racing charges $250.00 for balancing the reciprocating assy. Remember some old school machinist's wisdom.., Skilled labor isn't cheap...and Cheap labor isn't Skilled.
HR20