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I am changing the transmission and rear end in my 53 to a OD trans. and a 3.90 rear gear. Anyone know how long the drive shaft needs to be? I haven't torn the truck apart yet just getting everything together prior to the tear down.

That will depend on a couple of things but it sounds like you will have to have a new driveshaft made. Is the new tranny a manual or auto? Is the rear axle stock (torque tube) or has it been swapped?
There is information on the Empire Driveline site that identifies how to measure for a new drive shaft.
I suggest you call around to a couple of shops in your area and see what they say they need in the way of dimensions to get you a shaft. They are in the yellow pages.

You might be able to measure from tranny yoke/input shaft to the axel u-joint mount and then get the right length axel from a boneyard. Then go to a driveline shop with the yoke from your tranny, drive shaft and info about which u-joint you'll need to bolt up to the rear axel for balancing, installation of the new u--joints etc... You may even find a drive shaft with a yoke that has the right number of splines for the output shaft on your tranny!

Having built hundreds of driveshafts the only dimension I needed when repairing or building a new replacement shaft was the weld/weld dimension. Naturally if the rear end or tranny requires a different yoke that is another matter.

Also, FWIW, there are as many different slip yokes, stub shafts, etc as there are stars. So it requires not only the spline number but spline size, OD, length, etc so be careful when choosing used parts. Close only counts in handgrenades and horseshoes.

Well folks to calrify what I have is a 55 first series rear end housing, a earley 60's 3.90 pumpkin and a Borg Warner O D trans. I was hoping that someone had done this change over and could give me an approximate length of the drive shaft and what yoke to look for in the bone yard.

Chances are your gonna have to get one a little longer and have it cut to the proper length.I Had one done for my 55 2nd,and the guy who cut it down told me exactly how to measure for the length I needed.Go ahead and install the engine,tranny,and rearend then find a good shop to do the modifcation to your driveshaft.Your chances of finding one in a wrecking yard that will bolt right up are slim.