I think it is more like, with a single plane each cylinder has the opportunity to pull from whatever cfm the carb is rated for, with a dual plane, each cylinder is pulling from half the carb.
Your 351w should be 28 oz. imbalance but you can get whichever weight you need and it bolts on then if you prefer you can take it to a machine shop and have them check the balance between the old flywheel and the new, cost me $100. Then if you ever go zero balance take the weight off. You would probably have to change the throwout bearing to get a round face bearing.
Bought this one from levy, and have a mcleod on the other car i bought used. I don't think they cost over $1k if at all.
