As 8aaron8 said, the saddle setback and height in relation to the BB should be the same regardless of the STA. If both frames have the same top tube length, using the same length stem will cause the reach to be shorter on the bike with the slacker STA. Just set the saddle in the proper location and measure the distance from the tip of the saddle to the bars and adjust the stem length to match. It still may not ride the same, because the front wheel will be closer to the bottom bracket, but the fit should be the same.