Does the rear axle have a welded on rear cover or a bolt on rear cover. Also, if you can post a picture of the pumpkin, that might be helpful.
I had the same trials with my mystery rear end that you are having. I eventualy found mine to be from a 1957 chevy car. I believe that axles like mine were produced into the early `60s. So it is possible that your axle might look like mine. Mine has a welded on back cover. On the pumpkin housing, to the left of the large GM letters there is a casting number with 8 digits. Above this casting number, there is a letter and 3 numbers. This is the date code. Mine is A297. This translates to Jan 29, 1957. If you can find similar numbers on you axle, then it will be easier to indentify it.
Note: I has to scrub off 45 years of hard grime to find the date code.
