I forgot something. To change the rear main seal, you do have to pull the oil pan and the rear main bearing cap. The 235 has used 2 different rear main seals over the years. The 235 1st series from 53-55 used a rope seal. The NOS asbestos work the best, and are available from Stovebolt Engine Co. The 235 2nd series '55-62 used a 2 piece rubber seal. Its available at NAPA. The rubber seal is easier to install.
Since '55 was the crossover year, some trucks were built with the 1st series 235, and others with the 2nd series engine. Someone could have installed the wrong seal at some earlier time. If your engine has the 4" diameter water pump pulley, its a 1st series. If its got a 7" diameter water pump pulley, its a 2nd series. There are many technical differences between the two engines, so some parts will swap and some won't.
Have you made sure your motor is a 235? Some Chevy trucks came with a larger I-6 called a 261. It looks very similar to a 235, but its got a full-flow oil filter and other differances. Post the block casting #s to ID what engine you have. The numbers are on the pass side of the block, near the motor mount or the vent tube behind the fuel pump.
good luck
chip
