K should be October they skipped I. So, October 20, 1968. I'm seeing that it could be a 350 HP GTO engine as opposed to the 360 hp. Either way, yes should have been out of a GTO.
1968 400 Coupe, verdoro green, black vinyl top, auto 1969 GTO Judge coupe, Carousel Red, manual 1971 Trans Am, cameo white, auto 1970 Buick Skylark Custom Convertible 350-4(driver), Fire Red, white top, auto 1972 Buick GS Stage 1, Royal Blue, black vinyl top, auto