Use a solid 7260 race u-joint if you're not already using one.

The guy who built my custom driveshaft used them for my old street driven built 340-4spd Duster since my HP was way more than stock.

1970 FE5 Duster 360/904/3.91's SOLD 1973 TB3 SpaceDuster 340/4spd/4.10's SOLD Moparless for now but when the opportunity is right I'll have another one.