Wietse, just re-read this thread. I've built many 727's over the years, mostly for drag racing. I agree with dvw on almost every thing. The Transgo kit worked well on one of my HD street builds. Use a 3.8 lever( I'd prefer for your application, or 4.2 if you can find or buy one0, red solid front band, 10 or 12 springs in the front clutch with 4 red clutch frictions, if your stock steels aren't burned/scored use them, rear band should be OK, look for any burning or flaking, the stock sprag should be OK but look at the tangs so they are not bent, wavy springs are not broken, & rollers are not worn out of round. Set the clutch clearances on the tight side of service manual recommendations, especially the front clutch, you may need to get the snap ring selection(dealer or aftermarket) to get the desired front clutch clearance.
I didn't like part throttle kickdown on most of street vehicles because it sometimes kick down unexpectedly or not when I wanted it to. My preference is kick down only when I decide I want it. Good luck.
These are just my opinions.