heatercontrolvalve.com he does nice work and gives a warranty. Your only option is to go with a different core and an external control valve in your engine bay. He can rebuild it and you can retain all your factory appearance and controls.

http://s971.photobucket.com/albums/ae197/jon63c10
1963 c-10 shortbed stepside 427 BBC T16 HD 3 speed
1963 c-10 shortbed fleetside 348 "w" 1st gen big block & powerglide