A 350 can be installed. People do it all the time, but It requires moving he steering box. I don't know what your expectations are, but a good running 261 wasn't a bad engine. If it is a single speed axle, I would suspect that it was a special order for a fire truck or government vehicle. Two speed ratios were either 5.83/8.10 and 6.33/8.81 which is not that much difference. We used to use two tons in our business over 60 years ago and they would run 60 all day long. GM used their proprietary Eaton two speed that was vacuum shift. They were a headache when they were new and they haven't gotten better with age. Ford, International and Dodge used electric that worked. 400 series and Larger GMC used electric shift. If I were going change, I would look for an entire axle with electric shift two speed.. I have sold several vacuum 2 speeds for scrap iron.
