The stock valve body will work just fine as long as it is set up by someone that knows what their doing. A shift kit in a torque-flite trans is like wetting your pants in a dark suit, it gives you a warm feeling but nobody notices.
The only thing a shift kit does is raise the line pressure and has you drill holes in redundant locations. It also has you put in springs the are not the transmission's best interest.
If you want it to shift like a mule, have a race trans shop set it up for you. Otherwise just set the line pressure at 1. 250"-remove the accumulator spring- and set your rear band at stock settings and using a . 3150" spacer between the K/D servo piston and lever adjust it in till it stops and lock it there. remember to remove the spacer. You cannot adjust the bands with the pan on.
Anymore than this will require the trans being pulled apart and redone inside.
Look for someone that really knows torque-flites in your area. You may have to travel a little but, it's worth the trip.
I do my own, but, I am an old drag-racer too, so I have an advantage.
-dave-