Not exactly a primary source of information, but:
http://www.chevytrucks.org/resources/history_of_chevy_trucks.htm"Major engineering changes for 1963 included replacing the light-duty model's torsion bar front suspension with coil springs and replacing the former standard six cylinder engine family with all new 230 cubic inch and 292 cubic inch sixes. The former 235.5 and 261 cubic inch sixes remained the standard engines for K10 and K20 pickups only. A restyled grille freshened up all pickup's front appearances.
The most significant improvement for 1964 was the change from the dogleg windshield to the conventional curved type. It provided easier entrance and exits for driver and passengers and looked much better too. The 230 and 292 six cylinder engines continued as the standard engines for all 2 wheel drives and beginning in 1964 for all 4 wheel drive pickups also. The 235.5 and 261 sixes were dropped. The 283 cubic inch V-8 remained optional for all C/K pickups. The El Camino returned after a four year absence. It was built on the Chevelle's 115-inch wheelbase chassis."
