Steeper seat tube, longer stem?
 

My Kirk has a 72.5* seat tube angle with a 72.75* head tube angle and a 58cm top tube. I'm building up another frameset that has a 74* seat tube and a 73.5* head tube with a 58.5cm top tube.

I remember reading somewhere that you need a longer stem with a steeper STA. The Kirk has a 12.5 cm stem, so what length would I need for the new one to keep the seat to bar reach the same?

Thanks.

You would only require a longer stem if your actual saddle setback decreased which is, or can be, independent of the STA depending on your seat post choices.

Thanks.

backwards...
 

The .75 degree steeper HTA might give you 1-2mm more reach and the 1.25 degree steeper STA will give you about 12mm more reach, so you'll most likely need a 110mm stem, once the saddle is moved to the same position relative to the BB. If you don't maintain the same saddle position, then it can't be said what stem length would be required.

No, it's the opposite.

Compared to your Kirk, you're getting a steeper STA which using the same post with the same saddle position relative to the seatpost would move you forward on the bike. We're talking 1-1/2 degrees here, so you're going to move forward over the bike 21-24mm's if you have average proportions. Now, since you've moved forward, that new position may not be good for you, so you may want to use a seatpost with a longer setback (not that you'll find any longer than a Campy) or you'll slam your saddle all the way back and make up the difference on the rails. If you used a straight post, ala Thomson, you'd move even farther forward. Now if the new forward position works for you, you'd need a longer stem to make up the difference between how much farther forward your saddle is (and your position on it) minus the additional TT length you'll have. Since you have a steeper HTA, which puts the stem farther forward yet, it will also add length so you subract that from your new setback. Clear as mud?

if your set back stayed the same, the effective top tube would increase and the stem would decrease to result in the same effective reach. You may need a seat post with more offset to achieve the same set back as the kirk

To estimate more accurately, additional information is needed -- like your frame sizes, and where your saddle and bars are presently located. To get really accurate, you also need the stem(s) angle.

Assuming that you plan to keep the same saddle-to-BBKT position, the difference can be a couple of CMs (based on your top tube length I assume it's a big frame).