I know that the yoke would be different but will the U joints be the same and can the driveshaft be used?
You will need to change the driveshaft and you might as well upgrade u-joints while you're at it.
Yeah the 318/904/8-1/4 shaft will be longer. I got lucky on my 66, it came with the poly 318 but had a 727 and 8-3/4 so I could just stick the original back in.
http://www.dodgecharger.com/forum/index.php/topic,23738.0.html
The above thread should answer some of your questions. Basically, the driveshaft you have is approximately 4 inches too long and uses 7260 yokes. 7290 yokes would be preferable. Also, probably would be better to have a 3 inch or little larger diameter driveshaft. I would have one made up by someone locally---------or you can buy the one from Mopar Performance that you cut to length and have the weldable yokes welded on and then balanced. Check your yoke on your 8-3/4 to see what it is. Measure the inside dimension between the "U". If it's 2-1/8 inches its a 7260. If it's 2-5/8 inches, it's a 7290. And there are two different 7290 yokes. One is fine spline and one is coarse spline (in case you buy one for the rear end)---you can tell the difference by the attaching nut size. The transmission slip yoke in 7290 version can be found at most any junkyard on a C body like a New Yorker for example.
There is also a conversion U-joint that works with the 2 1/8 by 2 5/8, so you can use the larger driveshaft with the smaller yoke, or vice versa.