Hey Don,
I had a similar problem with my 34 Ford 4dr. The original builder had 3 senders with the last one attached to a right angle adapter. They were for Computer (LT1), Speedo, and Cruise Control hanging off of my 700R4. Needless to say (but I'll say it anyway... ha ha), is that the speedometer Gear Cover warped with the added weight and leaked. No amount of O ring replacement would resolve the problem. I resolved the problem by replacing the warped gear cover and going to a single speedo sender going to the LT1 computer. The computer has a wire coming out of it for speedo pulse that I tied to the Speedometer and the Cruise Control. It resolved the leaky trans and everything gets the speed pulse it needs to work properly.
Jerome