I almost never do such repairs (at least not in the last 20 years), so I can't give you play by play. I think the 420 has the starter in the most advantageous spot, but you will have to get a long way under the car and possibly go over the trans to get to the top bolt.
None of this should be done on a jack!!! I swear it wouldn't have stopped me in my formative years. I replaced the differential in the parking lot behind a Miami Beach Hotel on my 56 Chevy with the 350hp 327 in about 1968. Did another on a Sunday with the luxury of a lift rented from a gas station in Haines City. It broke on my way back to school in Gainesville 1969. Did about 20 or more differentials as they wouldn't take the power.
It wouldn't have stopped me as I would have figured some safe way of getting the car off the ground. Now being older and lazier I'd get it towed.
Bosch Master, ASE Master, L1
33 years MB technician