Whilst I have flown my J3 from the front seat (with a pax in the back) I much prefer to fly from the comfort of the rear seat.
I am not particularly tall, but I really didnt enjoy the experience of flying with my knees seemingly under my chin. That is without the gymnastics required to enter the front seat in the first instance.
With regard to the legal aspect, I am fairly certain that there is no designated P1 seat. After all, an instructor would occupy the front seat in a J3 / L4 and be in command.
SRD