Its not the two bolts used to attach the axle to lower control arm on the wagon
that controls the pinion angle, its the relative length of the upper and lower arms. Actually the pinion angle changes quite a bit, based on the loads put on it,
the ride height, and turning forces, etc.

I am assuming that you are using the original axle's control arms?
So the issue is how to attach the lower arm, with a bushing with two holes,
to the bracket on the 1800 rearend that has only one. Assuming that is so,
I would suggest modifying the lower bracket on the 1800 rearend, so it can mount to the original mounting point on the control arm. It should be fairly easy to
modify these brackets so that the location of the mounting holes are in relatively the same location as on the wagon rearend. Doing it this way you won't change the geometry. Otherwise, you will run a risk that when the car is actually on the ground and out on the road that the pinion angle will be "wrong".

I should qualify this advice by saying that although I've done this swap on 122 sedans,and have completely changed the geometry, number of control arms, mounting points, etc., on 1800s, I've never actually done 1800 disc brake swap on a wagon.

John
V-performance.com