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I ran a compression test (mildly warm) which returned: 125, 115, 115, 112. Probably need rings.
For sure it needs new rings
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What other areas should I be checking for wear? (end play and such?)
The only other common area of wear is the valve guides. End play wear is rare in a Datsun A-series. But still, you should check all the items listed in the repair manual. Use a Chiltons, Clymer or Datsun factory manual. Only $20 on amazon.com or $10 at half-price books (e.g. a Datsun 210 manual). See
FSMEven though the bearings may still be OK, spend $5 on plastigage and measure the bearing clearance.

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How hard is it to install a new main transmission seal and rear engine seal?
Easier than doing the same for an MG engine. Any repair manual covers this. You can get any B110, B210 or B310 repair manual and it will cover the Datsun A-series engine.