Slant six on boost - upgrade the driveshaft?

1966 Dodge Dart with an A904 and a boosted 225. This combo may make up to 300 hp and 400 lb-ft at the crank at full boost if I go for that; may run it at less. I'm swapping in an 8.75" rear axle since I didn't think the 7.25" would survive too many hard launches. What are your thoughts on the what to do with the stock driveshaft? Shorten the stock part and stick a bigger U-joint on the back, or upgrade to something bigger while I'm making changes?