speedometer gear replacement

If you have the style that came out in 66 & was used up until I-Don't-Know-When, the adapter has a special O-ring. It's red & made of a special material that is supposed to swell up once it's in contact with the trans lubricant to give a better seal. (This is the big O-ring, about 1 1/2" in diameter. I don't know about the smaller one insite the gear.). I needed one & was told by a Mopar dealer that I had to buy the whole adapter with the O-ring for almost $20. I tried using a regular O-ring but had trouble finding one the right size & it always leaked, even after I gooped it up with a bunch of heavy grease to help it seal. I ended up ordering the correct style one from a local Pronto Auto Parts. Don't remember the price but I think it was well under $2. I ordered a spare just to have around.