Gregger,
There was a 21 speed Hydra-Matic (that HrL refers to above) that was available in late in 1955 in GMC big trucks (I do not know if it was available for Z-series 54/55 GMCs). I have only seen it in a manual. It would be interesting to see any version of that Twin Hydra-Matic.
The 302 engine mentioned above looks a lot like the 248 and 270 engines that were more common, especially on smaller trucks. I think that the 248 was standard on small 54/55 GMCs (its production ended in 1955), with 270s and 302s more common on larger trucks (the 302 might have been standard on late 55 Blue Chip 450 and 500 series GMCs).
The engine serial number, just to the rear of where the distributor enters the the block, starts with the engine size (often with a letter before that number).
