Driveshaft Strength ?

I am building a 74 Dart Sport that was originally a 318/904 car. I have installed about a 650 hp 440 with a 4 speed and dana 60. I need to have my driveshaft shortend a couple of inches. My question is, is it worth it to have it shortened or should I buy a stronger piece?

If youp have an honest 650 HP and are not bench racin with that figure, why in the world would you want to go the cheapo route on a drive shaft?