If that cup has the correct dimension to be a snug press-fit then just lightly grease it, I would not use LocTite of any color/flavor. But surprised that since ISO (Campy) cups are supposed to be 30.2mm and JIS are supposed to be 30.0mm you seem to have the opposite situation, but could be just the luck of tolerances. Plus the bottom of the lower headlug sure looks like paint damaged due to something that could have caused the inner diameter to be enlarged (I'd try taking a bunch of measurements and averaging them just to satisfy my curiosity). But what works is what works, if you need a 26.4 crownrace for that 600 (and happen to have a 27.0) they DO exist but you'll have to "go fish" (and I do not have such a fish, myself).Best of luck, this should be a fine ride when you get it all sorted!

BTW, GOOD idea to use a press and not hammer the cups but if you don't have $$ for a proper tool you can make a cheap but effective substitute with a length of all-thread (threaded rod) and a stack of washers and some nuts, see Sheldon Brown's site for pictures, Slightly clumsy to use but very much an improvement over hammer/blocks of wood!