I agree with cut or drill off the head, then remove the flexplate. You will need another bolt, and they are quite special. I recall you can still buy them, albeit >$10 each. Check if they are 1-use only (torque to yield, why they are necked-down?).

Re changing the rear seal, search for Rollguy's post w/ photos where he did so. He left the crankshaft in but loosened the main bearing caps enough to drop it to get the rope seal out. M-B (thoughfully?) put a locking pin in the seal groove so you can't just pull a new graphite rope thru like people would do in a 1960's Chevy V-8. Rollguy's photos will show if he left the flexplate attached.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans