Measure compression on all cylinders. If good (say >380 psig all), I would leave the head on and just replace journal bearings (rods and mains) since cheap and easy. If bad (<300 psig any) you must pull the head (unless just due to bad valve adjustment). If you had large blow-by, replace the cylinder liners - then machine shop hone cylinders and deck the block. Then piston rings and might need some used pistons if any have chunks missing at the ring grooves. Have the head checked and/or refurb'ed while off. Look for my post on removing the liners (easy, flat screwdriver peels cast-iron away from block).

Either way, replace the oil pump nylon tensioner while in there since cheap and cracks. Replace the front crank seal (easy to mess up crankshaft notches for damper so read first). You can replace the rear rope seal with crankshaft in if you lower it slightly (see Rollguy's post).

In sum, the most important thing is diagnostics first. Don't jump into fixing something which isn't broken or you may needlessly mess up a good engine.

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