What
oil and weight are you running? 20 psi is really not bad at idle. The rule of thumb is 10 psi/1000 rpm. I'd run it until it really is tired.
If you are running the original pistons after a rebuild, then you must have oversized rings unless the cylinders were not bored which would be highly unusual. At the next rebuild you will have to go with new pistons, but the 289 doesn't like to be bored much over .040. Worst case you would need a new crank, rods and pistons.