I am rebuilding my 289 and have had no problems with the driveshaft previous to this but should I have it rebalanced anyway seeing as how the engine is pretty much new

 

No reason. Just put in new u-joints.

 

i agree, you might want to spring for an alumimun one though.... maybe reduce the rotating weight and free up a few extra hp?

 

289? Is this a 60's Mustang?

It's pretty much up to you what you want to rebuild or what you want to leave alone on a redo, restoration or rebuild.

A few factors that may help you make a decision.

Is the shaft rusty, bent or dented. Did it shake before. Tall rear gears keep driveshaft rpms low. Balance is not quite as critical. Mustangs are lucky to have short driveshafts. Bigger cars with long driveshaft become more critical.