Hi Rani, I went through this exercise (rebuilding my 727) last year and collected a lot of great info in this thread. I was new to the whole thing, so it's written from the perspective of really trying to understand the details and differences.
A727 Rebuild kits redux - Rebuild kits everywhere (62-70 vs 71+)
This is a excerpt (from Tracy) that may be more specific to your questions.
"Basic differences, the 66-70 had a 1/2 wide reaction shaft bushing(the one that's pressed into the front clutch hub) and different reaction shaft accordingly. 71-up had a 1" wide bushing, much more stable and lasts longer. Due to more stability the reaction shaft sealing rings also last longer. 71 and up also had a revised seal in the front clutch that lasts longer and seals better. 71-up had part throttle kickdown. If it's a street car it's a nice feature because torqueflite's with a shift kit are notorious for the kickdown not working good enough. If your like me and like to manual shift most of the time it's no big deal. The part throttle kickdown is a part of the valve body. I've heard you can bolt a part throttle kickdown module to a non part throttle VB but tried on one and it wouldn't fit. There are some variance in valve bodies so maybe you can on some but not others?? Again this is just a creature comfort item, not at all necessary.
BTW: To upgrade your trans. to a 71-up style all you have to do is swap in the front clutch drum and reaction shaft from a 71-73 model. Make sure you get a drum that holds 4 clutch discs, some only held 3. If you decide to upgrade to the newer stuff any later than 73 and parts interchange is hit or miss as they revised the rear clutch drum a few times and it may interfere with the reaction shaft. You could always swap out the rear drum assy. as well if you have the money. I do this on all serious high perf. applications but lower hp daily drivers I don't worry about.
The kit your looking at at CRT is a great kit but at 500 hp upgraded frictions aren't a must. I've used the kit Tony recommended and up to about 550 hp their good enough IMO, plus their more affordable.
Unless you get it for the same price there's no need to upgrade the rear band as it's not a shifting band, just a holding band. Since your going with a flex band up front I'd use a 3.8 lever. I use a 4.2 lever with solid bands only. If your using a high stall converter (2800 stall or above) another mod I do is to put a blocker rod under the accumulator. Firms up the 1-2 shift and reduces 2-3 shift overlap. Use 12 new springs in the front clutch pack. Yours may already have 12 but they loose they stiffness over time so need replaced. "
AlV
