How should I install a door that is not pre-hung and has a custom jamb?

Question

I have an antique restored interior door with a transom for which I have to make my own jamb due to the height of the side jambs (big boxes sell only 80" door jambs). Should I

A) assemble the jamb and hang the door on it outside the rough opening before fitting it in (basically pre-hang it myself) or

B) assemble the pieces of jamb inside the rough opening?

I am inclined to think A because how else do I know how much shimming I will need. However, one problem I see with that is how to assemble 1x planks of the jamb at right angles around the door and set the hinges in correct spots. It seems like I would need a firm frame to support the composition.

Answer 1

I like B, because the most critical jamb is the hinge side. Once its plumb, with shims behind the hinges, I rout the hinges in place with templates.

The other 2 jambs are "loose" until the door is hung. Then, with a helper, the gap around the latch side is dialed in, starting at the latch.

The top jamb is done last and isn't as critical, structurally, as the other 2.