You might try contacting
Northwest Transmission Parts. The have parts for old 4 speed Hydromatic, and are very nice to deal with. I've bought parts from them for my 3 speed/overdrive, they were very fair on their prices.
If you do decide to install a later automatic, Langdons and Buffalo both make adapter plates to bolt a late model Chevy bolt pattern automatic to the 235. I would personally suggest the 700R4 for your application, the 200 is way too light for truck use.
