There are different sticks for different S/N PA18's, I think the first 18's had perfectly straight sticks?
If the rear stick hits the seat back, the elevator cables are useually installed incorrectly. By lengthening one, shortening the other, you change the stick position.
When making up new elevator cables, make them so that when the front seat is full back, the tension is correct, and the turnbuckles are tightened properly, the rear stick does not touch the seat back with the cushion installed. This takes some extra work, but DO IT RIGHT, it's important.
And give it some extra clearance. Remember when you are sitting in the front seat, expecially in a high angle of attack, your weight is pushing the seat back springs back a lot. I was test flying a PA18 once, doing stalls/spins, and in a power on stall pushed the stick full forward to get out of a high angle of attack, and nothing happened. The thing stayed pointed at the moon no matter how hard I pushed on the stick, till I felt something hitting me in the spine, remembered the rear stick was in, I slid forward in the seat, allowing the rear stick to push the seat back forward and the nose came right down. In normal flight it was fine, but at a high angle of attack, my big ass pushed the seat back cushion back far enough to severely limit the stick travel. Also, don't put in a seat back cushion with one of those big storage pockets on it, if someone puts the rear stick in with a load of junk in the pocket, they will have problems. I (since that test flight experience), always make a point of checking the rear stick-seat clearance on annuals, and before I fly a Cub that I have not flown before.