Tabbing sleeves to the first and third stringers with less than a extra, less than full depth stringer added to each would be my suggestion. This is similar to what is on Blitz. This makes these stringers appear as very wide stringers. FWIW, the dimensions on my boat are as follows:

1 31/32" OD tubes 20 7/8" OC
Front tube is 8 5/8" center to center measured from the middle stringer
Aft tube is 12 3/4" center to center measured from the middle stringer