Did a gear swap from 2.56 to 3.73, now the trans doesn't shift into 3rd. 1-2 is ok. Adjusted the stop on the kickdown cable, but no change so far. Going to retry. Going to double check vacuum lines next. Any ideas?

I know when I did my gear change from 2.78 to 3.73, my th400 shifted into third really fast after second gear, and the engine was screaming (close to 3000 RPM) at about 50 MPH. I take it, that it was shifting fine prior to the gear change and you have checked the fluid level? Yea, make sure your vacuum line is hooked up to the carb (ported) and no vac leaks and if good check the modulator.

I moved the kickdown stop 1/2" forward, and now it shifts. Not a very positive shift, so I moved it 1/4" more and have yet to test it.

Yes, was shifting fine b4 the change and fluid good. No vac leaks, 12 at idle, and hooked up to (I believe) ported w/ brake booster.

If the brake booster is hooked up to the back of the carb and it is at the base of the carb that is manifold vac since it is below he butterfly's on the carb. I don't personally know if it makes a difference, but I have always seen it hooked to ported vac which is towards the top of the carb.