You really can't depend on bare aluminum for good measurements. Tape 2 pieces side by side of flat black electrical tape (or shiny black and scotch brite it real well) and put it on the bare aluminum. Aim your temp gun a couple of inches away from the tape and you will be good to go.

67 Coronet 500 9.610 @ 139.20 mph
67 Coronet 500 (street car) 14.82 @ 94 mph
69 GTX (clone) - build in progress......