Not being fast, and being old, and valuing my comfort more than looking sporty, I sit bolt upright on a triple coil-spring Brooks B73 saddle (SJS had it on sale for under 50 quid a few years ago, and I decided it was worth an experiment -- never looked back). The On One Mary bars are in the generic style called "North Road", and the best bars I ever had are on my bike now, black no-name North Road steel bars taken off a bike a dealer sold to someone who wanted more fashionable bars, when I just happened fortuituosly to be standing there. The ergonomics of these swept-back bars, together with the knees-up position of sitting upright, is such that height of handlebars really matter more than length of top tube. So ask Thorn not to cut the steer tube, and to supply the frame with a Problem Solvers microadjust collar; this sits right on top of the head tube in the place of the stem, which is then liberated from holding the headset together, and can be slid up and down the exposed steerer tube, which is only sawn when you have the height finally sorted. The alternative to the collar is a set of wide and narrow spacers. You then adjust the effective length of the top tube with the stem. An adjustable stem will normally give you a little height change and more length change, which is the wrong way round for sitting upright but is good enough for fine adjustments.
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On one of my bikes, a Gazelle Toulouse, I have a toollessly adjustable stem which lowers and raises the handlebars, and also lets them rotate, so that for a three second adjustment I can lower the North Road bars, move them forward, rotate them, and achieve an aerodynamic flat back position much like with drop bars. I wish I could buy something like that commercially, but the Switch stem is proprietary to Gazelle, and available only for old-fashioned threaded headsets. Works a treat, though.
