In case it's not obvious when looking at it, generally speaking we set it up so that the larger Double-D shaft (1" DD) is towards the top and that u-joint/yoke attaches to the steering column.
The smaller section of DD shaft (3/4") would then be at the steering box.
If this is for the '77, you would see markings on each yoke indicating what size spline it is. The upper would be a ".75 x 36" (for the 3/4" x 36 spline shaft) and the lower would be something like ".8125 x 36" (or whatever the decimal alternative is for 13/16") to match a stock steering gear box.
So that's a starting point. Yes, for the length you simply slide it in or out to get the proper length based on your specific setup.
Depending on what box you have, there is usually either an existing detent or even a full slot that you can use for the set screw. If not, definitely dimple it with the tip of a drill bit.
Same for the upper in some cases. If this is a '72 or earlier model shaft you have to make your own, but if this is your '77, then there might be something there already. If not, definitely give the set-screw something to gain purchase on by dimpling the shaft.
Other than that, the hardest part is sometimes just getting the old stuff off that's been stuck in one place for 38 years or so. When we did my friend's '77 though, the old stuff just came right off with no fuss whatsoever.
Now's a great time to make sure that your steering box is on-center and your steering wheel matches that. It probably does already, but while you have things apart is usually the best time to verify other things.
See? Even with simple stuff there is the snowball effect!
Good luck.
Paul
