The "easiest" way to get wing tanks would be to order RV-3B wings (quick built still available?), drill out and replace the main spar bulkheads in the fuselage and mount the new wings with tanks.
Another possibility would be to convert the wingtips into tanks. I think that someone used to produce such.
Don't do what I did. My RV-3 project came with wings mostly done. In order to do the main spar modification I had to drill out rivets and peel back bottom wing skins. Because I then had access to the inside of the wing I pop-riveted closed the holes in outer and root ribs, drilled holes in top skins for filler cap flanges and coated the inside of each bay with angelhair (thin fiberglass) and epoxy, and pop riveted main spar lightening holes closed (with epoxy). Worked great for a while but with repeated hot/cold cycling from sitting outside all the time in the Florida weather (and possibly from ethanol in the MoGas I used) they started to leak. I anyway mostly used the 25 gal fuselage tank, and really only used the wing tanks for flight to OSH and back (taking care to use the fuel in most leaking wing tank first). OK, you can all stop laughing now!
Finn