im thinking 12 or 13-1 will be too much. about the time you lace into it a few times you'll be mopping up a huge mess. and i think i read you have a 305 and want to make it a 355, that will require a 350 block @ .030 over bore. not even sure they make a 13-1 piston for a 305, and stock oem heads will not be a good choice for that compression. it would run but be very very restricted. probably would hurt the motor eventually also.