23 degrees is nowhere near enough at 3000 rpm. Where are you getting your procedures?
Set the initial timing in Park, with the vacuum line disconnected, and with the vacuum leak plugged, of course. You are going to have to adjust your idle speed back down, and readjust it if moving the timing moves the idle speed, then recheck the tiiming.
It sounds like you "originally" (your last paragraph) did it correctly. Do it that way again . . .
Dieseling can be something as simple as not enough octane for the car. My GTO would diesel with 87 or 89, but not with 93 octane gas. If your engine is still original, you may be pushing it on pump gas anyway . . .
If memory serves, that engine was rated at 10.75 to 1 compression (someone will correct me if I got it wrong) and really works out to 10.25 to 1, which is still pretty high for iron heads.
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Badder than old King Kong
And meaner than a junkyard dog
-Jim Croce
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