I think visually it's a little deceiving because the Saga has a seriously fat head tube. I think the circumference is 1-2 cm bigger than the LHT head tube. The sloping top tube could have something to do with it too, but I think it's just the freakish head tube.

As for sizing the Saga, I'd go for the equivalent of the LHT. Honestly these two bikes feel exactly the same to me sizewise, despite some minor variations between the frames. I am 5'10.5" with 31.5" inseam and if the size 56 Saga were any bigger, I'd have to go a size down to be comfortable. I have a long torso too, so I went for the Saga with a longer top tube. Basically, if you feel too stretched out on a 54 cm LHT, I doubt a size 54 Saga will ride much different. You'll be more upright on a smaller frame too.

Either way, you have more than 2 cm of play between different stems and moving the saddle fore/aft.