In the 6 ACE route

 Go to YI1, get a powerup in reserve and drop it so it sets $EF to 19, then get another powerup and drop it so it sets $EE to C7. Kill yourself and get back to YI2. Do everything normal, except make sure to kill the red shell after the message box and the 2 naked koopas. Do all the shells, up until the slot 7 one, where things get a little weird. You have to first kill the slot 9 koopa, do the turn around and the shell kick upwards, THEN spit it where it needs to be. You'll also have to use the fireball from that last koopa during the swap, since you killed the other red koopa. After you spit the shell, just continue and do the swap