While disassembling I found this? it was stuck between the case and the sunshell.

What i this?

 

a dampener spring. it is supposed to go under the center support snapring against the case. it is to help keep the metal center support from beating the aluminum case to pieces "dampens". Definately install it back in during assembly or the case will be damaged over time. install it with grease (it fits against the case) before installing the center support and snapring. make sense?

 

center support? Sunshell? Low & reverse?

 

wholly crap. You didn't mention it was an old B&M case. Those wear the center support lugs of the case real bad. you probably need to upgrade to a bolt in style center support or the lugs of the case will break off causing the snapring to come off and clutches in the rear of the case to not apply at all. throw out that spring and install the correct part to prevent the case from being a throw away transmission!!!
the upgraded support can also be fitted with a wide low roller sprag from a 700r4 if ordered that way also! I've seen way too many of these cases damaged from the hard hitting B&M kit. Don't get me wrong I love the shift, but some preventative measures should have been taken on a "factory" b&M build from them. One of my good friends bought the same unit from them, and with less than 200 miles it broke out the case there making it worthless. He's not the only one-trust me. I'll try to attach a pic of the upgraded support.

 

Its not....thats just a pic I found.
I have a 350 long tail from my 73 c10 pickup

 

i'd still check the case lugs regardless. they wear out and complete failure of the unit will result if you re-use the spring with a worn out transmission case making it a throw away unit.

 

Are you reffering to the Band? This holds the band?