Does the whine happen with the clutch in and the truck stopped and not moving? When the truck is stopped, only the throwout bearing (and pilot bushing, if you want to be technical) is spinning when you have the clutch pushed in, so that will tell you if the throwout bearing is also loud. (BTW, you should have about 1" of clutch pedal travel before you feel it touch the throwout bearing and become more firm.)
In my video of my truck, when I'm not in high gear you are hearing the transmission gear whine--pretty normal for that trans (light duty trans are quieter). BTW, here are some pictures of the Muncie SM318 that most of these trucks came with:
http://67-72chevytrucks.com/vboard/s...d.php?t=574428 The later Saginaw will be much larger and have seven side cover bolts.
If you were actually in the truck with me, you could also hear a slight whine from the worn ring and pinion when cruising in high gear. Not a lot, but enough to hear over the (fairly loud) exhaust at times (I do have decent hearing). So, I'd say at least part of your problem is ring and pinion damage/wear. One possibility is that the guy working on it before might not have really known how to set one up, and left the pinion slightly loose when he changed the seal. If he did, the pinion bearing/pinion will also be slightly loose as a result, which would make it whine.
So, I think at least some of what you are hearing is the ring and pinion. It may or may not really be a problem, depending on how loud it is, and why it's loud to begin with.
If you have one of the new phones with the built-in cameras (or know someone who does--they pretty much all do now), just set it on the dash or something and have it recording for a minute or so while you are driving (preferably while you also work the throttle to see how the sound changes)--all that's important is the sound.
Youtube/Photobucket/etc. is pretty easy to figure out, but you could also text me the clip to my phone if you don't want to do that. Just let me know and I'll send you my number via the private messages.
Hope this helps,
Chaz