If you want another option, check out SMR transmissions, google for their website. They offer a shorty overdrive bolt on unit for the 727 & 904. Speak with Pat Storey, he's knowledgable and a good guy.
On the TH2004R, there's a thread on this site with the transmission going into a 68 cuda. Lots of real good information and photos. The TH2004R does have a decent reputation, but to keep it together, $$ has to be spent. GM never used the transmission behind anything that had much in the line of torque or HP.