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You can't put the front of the saddle in front of the BB centre without getting thrown out by the commissaires, so you're only talking about shortening the front centres. You don't need a steep head angle for that, you just need a short top tube and a long stem.

having the same front center, same trail, between these two examples:
-steep head angle + long top tube + short stem;
-slack head angle + short top tube + long stem;
what is the real difference?