The picture looks like a T89 which is very rare. The common one is the SM318 front section with a BW T10 overdrive. Both are .70 overdrive. BW published an overdrive manual in the 50's. I am in Germany at the moment. I can scan it and send it to you when I get home. If you pull the shift cover off and it looks like a Chev transmission, it will be the the SM318 front section and the book for the SM 318 will work. If it has brass synchros on second and third gear, it will be the T89 which was also used by Ford. I have the Ford books. Overdrives have always been my favorite. You can lock out the overdrive and have the fun of a 3.90 and let it drop into overdrive on the highway and have the benefit of 2.73.