what would cause a harsh 2-1 downshift on a c6? i just rebuilt mine and as i come to a stop it kinda bumps as it returns to 1st gear. only when i slow down and just before a complete stop it clunks the driveshaft as it returns to 1st. all the other shifts are fine and maybe its just the way it is but i thought id ask. please tell me your thoughts...

 

Try adjusting the low band a little. First one way, then the other. count turns. Don't try less than 1 turn free play after full tight. I don't remember which adjuster is the low one, it should be in the rebuild book.
You can get strange results if you put the wrong servo spring in the wrong servo, or if the cover doesn't have the drain lined up, or if the link on the end of the servo rod is not positioned properly between it and the band, or the link band to case is wrong. Also make sure stack free play is right according to the manual, and change shims behind the pump until it is.
Did you wash the valve body in a parts washer with Zek? The pros I worked with had a $3500 parts"dishwasher" that they would wash everything of a transmission overnight in, especially the valve body completely disassembled in a little mesh box. If there was any burning or sludge before rebuild, it causes the spool valves to stick in the holes, especially cold. We tried at work (snack food plant) to clean spool valves (4 way 2 position spool valves mostly) so they would work again reliably after they stuck up with vegetable oil. NOTHING worked. With 60's fords, we have the luxury of taking the valve body off and apart and putting in a frying pan with a little something; GM mechanics have to put the whole case in a $3500 parts washer to get the valve body clean. I've been meaning to try electrolux at 180 deg for 12 hours on a valve body, but haven't done it yet. The electric frying pan is too lost. I have 3 C4 valve bodies I bought used, and none work quite right cold. the valve body I'm using now is freeing up now that I am using Dexron- 3 years of clean Type F fluid did nothing to improve things, but dexron 3 is half detergeant.
After you try adjusting the bands, and the modulator (you did buy an adjustable modulator didn't you?) Sometimes the governor needs a different spring, this is the weight in the tailshaft area. It is the hardest to do, so it is the last one. Good luck on finding different springs, too, transmission shops have a box of them but I don't know where they come from, except the transmissions they disassemble that they bid but the customer abandoned the car rather than pay the inspection fee.