Take your valve cover off and on the driver side about the middle should be your casting date. The 848 head entered production for the 56 model year so the Wiki is wrong on the 55-62, its actually 56-63 (1963 there was one single application for a 235 motor, a NAAPCO 4 wheel drive, otherwise the new 230 style motors).
If the date code on the block is indeed December of 56 then it is absolutely a 57 production year block as the 57 production did not have the early and late designs like the 55. 55 heads would have been the earlier 913 head. If the head casting date is in that range, you might have a nice matched motor with its original head.
One thing to keep in mind, the 55 913 head and the 56 848 head used the small temp sender port. The 57 and up used the larger temp sender port. So while both of those heads will interchange with one another, the temp sender ports may not be a direct swap.
