Driveshaft length

I'm just about ready to get on the road with my 1970 Duster street/strip car. It's a 600 hp 440 with a 727. I need a driveshaft and have been frustrated by the $400 price of new ones. At my HP level, I want to make sure I've got something strong, though. I don't have a yoke in the transmission yet, nor a flange on the 8-3/4 diff. (there's a Mark Williams coupler on there that I plan to remove). The measurement from the end of the trans. output shaft to the end of the pinion shaft is 48.5". I'm thinking this 47.5" one on ebay might be a good bet (with a yoke in place, I'm figuring I'll need a slightly shorter shaft than my measurement). It's one helluva good price, and all I'd have to do is fit the correct yoke. Thoughts?

http://www.ebay.com/itm/DYNOTECH-DRIVE-S...18f&vxp=mtr

I would wait until everything is attached and you get an accurate measurement of what you need.