Some other places to try that deal in parts for those engines
[www.bowtiebits.com] here in Wichita
[www.oldchevytrucks.com] Kansas City
Also, I would recommend having the damper rebuilt. The rubber ring inside cracks with age, and will come apart, damaging the radiator and engine. I have used the Damper Doctor, formerly Damper Dudes in Redding, CA
[members.aol.com]
Also, the dual pulley dampers were on the later model trucks as the early power steering pump mounted on the end of the generator. Don't know exactly which year they changed, but the production of the 235/261 ended in 1962. You should be able to use a damper made through 62. If you can't locate a dual pulley, one can be added to a single pulley damper.