The rear main used on the AMC242 is of two-piece construction, rubber over a metal core.
To remove it with the crankshaft in place, you'll want to get a short (about 4-6",) thin (about 3/16"-1/4") brass drift or punch and lightly tap the end of the upper seal half on the end. This should allow you to drive it out far enough to grab the other end and pull it loose.
While you're doing all of this, you should have the new seal soaking in clean engine oil - this not only allows you to install it easier, but also lets it "seat" more quickly (due to being more pliable.)
Slip the new upper half into place, and slowly push with the brass drift. NO STEEL SHOULD TOUCH THE CRANKSHAFT! (Yes, that's all caps. Yes, there is a reason...) Properly installed, the new seal should protrude only slightly on both sides of the crankshaft - it "crushes" against the lower half to seal at the ends.
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