Looking at the back of the '66's block, I noticed a smallish plug towards the right side, next to what appeared to be a freeze plug. Is this for coolant or oil? From the looks of the location and the contours along the side of the block, it looks like it plugs off an oil galley. Is that in fact what it is?

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

Yep.

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Official Cookie and Mater Tormentor.

Yes, the pipe plug is the main oil gallery.
The "freeze plug" (expension plug) next to it, covers the rear of the cam.
DD