John, don't know if these photos will help.  But, like you I only have between 3 and 6" of duct distance on the radiators.  You should do Ok with 20 sq inch on each opening with a good diffuser/duct.  Attached are some photos of my current ducts.  The openings are 18 sq inches each.  I have had one opening down to as little as 10 square inches - but that was a bit marginal - so opened it back up.  I have a generous exit area for the hot air including a larger 4" x 12" bottom opening as well as louvers on each side of the cowl.  So you mileage could vary - but Tracy has essentially the same size opening as well as several others.
 
Ed\