Tejas Steelworks has a crossmember that works with the 6L80. I have done two. One with an NP208 and the other NP205. On the 208 I used the factory crossmember but had to make a bracket to bridge the gap between the transmission and the crossmember. In the 205 the Tejas mount was perfect...
I also have the RPO code A28 and have the aluminum slider like TLambert posted. My felt runners are trashed and the window is missing the center aluminum uprights. I am in the process of restoring it if possible. I was able to remove it with the rubber surround seal intact. I also have...