Driveshaft question:

Bill, I don't have any exact knowledge, but I'll give my thoughts.
I guess you have to know that the shaft you have now is the correct length, before duplicating it. I'm thinking a new one w/ bigger u-joints would have a slightly shorter "tube", because the larger diameter u-joints have to be compensated for.
All that aside, there's a method for measuring for a shaft. It considers sufficient spline engagement in the trans and suspension travel. I think, when we had one made for our stocker back in '94, this is what we did. The key things are that it doesn't go too far into the trans when the suspension is at the point where the pinion yoke is closest to the trans, or have insufficient engagement when the pinion yoke is farthest from the trans. I'm sure you're aware of this, though.
I would prefer to make a new one to my own measured specs, as opposed to duplicating an old one that may or may not be the correct length.
I would check this theory before using it. Maybe the shop that builds the shaft could offer advice?