Not absolutely sure, but I think IF you have a cast crank you will also need external balance on the converter and/or flexplate as well as the damper. This is the case with 360's. I agree that a 360 damper isn't the same as a 340 one. FWIW '73 340's had cast cranks, but since your engine seems to have been hacked together, who knows what's in it now.I don't know what your plans are for this engine, but IMO external balance dosen't have any place on a performance engine. It's fine for trucks and everyday cars.