Is there a drawing for the ventral fin?
From the TC:
204. Edo 89-2000 floats with water rudder and seaplane fin (Dwg. No. 11030) +183 lbs. (+14.5)in accordance with Piper Dwg. 11031 (without spreader bars) orEdo Dwg. 89-180A (with spreader bars). Airplane Serial Nos. 12-1 through 12-3011,and 12-3901 through 12-3966, except 12-3943, 12-3945, 12-3947, 12-3949 and 12-3965, must haveadditional fuselage brace tubes (Parts Nos. 11026 and 11027 in accordance with Piper Dwg. 11031).
The fin is required in accordance with the TC unless the rudder centering springs are installed in accordance with the STC.
The centering springs do nothing more than to center the rudder while providing opposing force to the pilot's foot input in making control inputs. They will be satisfactory when the plane is loaded at a forward CG.
When the plane is loaded at or near the rear CG limit it will tend to fishtail more. In this condition more vertical tail area is required for stability. The springs will not provide stability in this condition. Even though you will be legal.
Where have all the fins gone? Some have been removed and left in the barn because they do get in the way during some ground handling conditions. Some having been removed while the plane is on wheels get lost when the plane is sold separately from the floats. Now 70 years later they are few and far between.
My long wing -18 clone on EDO 2000 performs best when loaded with a rearward CG. In this condition it will fishtail in turbulence requiring lot's of pilot inputs which becomes very tiresome. I installed a ventral fin from a Husky which was on Wipline floats when the Husky was converted to stabilizer fins. With the ventral fin my Cub flies beautifully, without it is uncomfortable.
Since you are in the Seattle area you might be able to install a set of stabilizer fins from a Citabria. The Seattle region of the FAA is very understanding in these matters. At least they used to be.
However you approach this you will be happier with more vertical tail area.
If you have altered your wing tips from the original Piper shape, the instability will be more noticeable.