Yes, you can slice the gasket around the windshield and rear window to get the glass out. That's the easiest way.
On the color, GMC different color choices than Chevy trucks. My first thought was your red was Chevy Omaha Orange, but it's definitely a red. The red on the cab where the bed was is a sign of the original color.
This LINK should show the colors available for 1948, although it's for 1950. The same red was available in 1946. A modern paint code for Pimpernel Scarlet is Martin Senour NAPA 98L-1014.
