If you feel like the bars should be brought back an inch and a quarter, thats almost 3 cm. If the bike was outfitted with a 120mm stem and everything else is sized perfectly then a change to a 90 stem would not be completely unreasonable. If however, the saddle position is so good that moving it a bit forward would be a bad change then the 56 mike may be too large for you. A 54 would be shorter by 2 of the centemeters that you are asking for. (The top tube length will end up being a more important fit parameter for you than the length of the seat tube).
IMHO you may want to try a 54 after all. 56 is a pretty big bike for a 50+ rider who may want to be a little more vertical. Just for example, I am almost 6' tall (with short legs) and I have come to find that I am much more comfortable on a 54 or 55 than my old 57.
I would recomend a frame that starts you out with a 110 stem so that you have a little wiggle room for later changes.
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