Who knows universal joints?

I am looking to replace the U-joints on my 1971 Plymouth Duster 340. It is a street car and I don't beat on it. I prefer Spicer brand but one parts site says 5-789X will fit, and says it doesn't. Does anyone know the Spicer part and should I go heavy duty or light duty that can be greased?
Thank you.

Buy the best U joint you can get now just in case you decide to get abusive with the car later
As far as the U joint it is either a 7260 Detroit (inner keepers) or the larger 7290 Detroit type U joint.

it'd either be the smaller 7260 which iirc is 2&1/8 from inside to inside on a pair of ears on the shaft or a 7290 which is 2&5/8" (I hope my dimentions are correct, measure yours) & likely that might account for the discrepency in the listings (showing 2 numbers). After installing on the bench confirm the yoke in front moves smoothly thru both planes of travel & the rear ujoint moves smoothly thru its single plane of travel (whack the 2 connector "ears" on each end of the shaft with a medium BFH as needed) & make sure the zerk is in compression. confirm no needle bearings fell down during the install. EDIT an eBay listing says it is a 7260 & 2&1/8" is correct & a 1.078" bearing cap O.D. MORE EDIT consider haveing the shaft balanced (or at least checked by a competent shop) that balances for HIGH RPM (some balance too low). You might talk to Doc

Thank you both, lots of good information in there. I think the Spicer 5-798X looks like a winner.