The 727 would be my choice. My Challenger 360 has the 727 in it, and back in 1971 when I dropped $3200.00 (sigh) for my new 340 Duster, it had the 727 transmission in it. The 727 in the Duster took a lot of use/abuse and never complained. The 727 was designed for heavy duty applications, while the 904 was used for light duty applications. The 904 can be made into a HD unit, but a lot will depend on your budget. The TH200 4R Conversion is also an option, however it doesn't appear to be a bolt in job. Another overdrive you might want to look at is from SMR Transmissions. www.smrtrans.com
Pat Storey is the owner there and a great guy to talk to.