If you havent started your build yet, set some time to watch my 700R4 and 4L60E "high perf" videos in each respective playlist. I create and publish them to quickly share large quantites of information specific to those transmissions, that would otherwise take much longer to explain. There's plenty of content that will give you ideas of what to do and how to do it. Obviously, feel free to ask questions along the way.Yeah "shift kit" is a buzz word. When I built my TH350 I didn't use any "shift kit". I just removed the second an high accumulator springs,made spacers for them and drilled the separator plate. That tranny is still in my Elcamino and has nice positive shifts that aren't abusive. If I can do that with this tranny then I see no reason to buy a "shift kit". This truck is my daily driver. My concern is longevity and speeding clutch apply will make them last a lot longer. Anything else I can do to increase longevity I'm all in.
The main areas of weakness of those transmissions are the TV boost, plunger/sleeve, pump, 3-4 clutch (under engineered in general and esp relative to engines these units frequently see today), band (usually too much clearance, too aggressive a hydraulic calibration, reuse of reverse drums with warped surfaces) and sun shell.
Focus your attn and resources beefing up those areas and you'll be good
The gear train/output shaft is fine, as is the input shaft and drum as long as they dont leak. Gear lube mods help with longevity and a Sonnax Smart Shell is a great bang for your buck upgrade. Pair it with the 03+ reaction shaft and you're set with that part of the trans.
The rest of the clutch packs arent overly vulnerable and easy to set up correctly - add 97+ bonded steel/rubber apply pistons and the fwd return spring to go with them and you're set. Later valve bodies aren't fussy and hydraulic mods are straight-forward...Also, remove the tcc check ball in the snout of the input shaft and install the 4L60E equivalent to get more TCC apply fluid to the converter clutch quicker...