Wow, now you have done it! A difference with the later radius arms would have occurred to very few. Hundreds of owners of high chassis cars with later axles will now be unsettled! Ian Williams recently established the difference as about 1 deg. I guess the spring assembly will reasonably accommodate this. Many obviously do. Some will consider the castor reduction significant.
As previous a card cut to 95 degrees held against the face of the radius arm nut, should present the other side horizontal to a spirit level. Assumes nut of uniform thickness and not over a spring washer.
My RN saloon has a Girling front axle with Girling rad rods.I did not want to lower the ball joint as this puts more torsion on the front crossmember and puts a larger eccentric load on the weedy centre strut. I bolted the front axle/spring assembly to the chassis with a similar amount of castor angle as my chummy and after cutting the Girling rad arm (just aft of the drop stamping carrying the spigot) vertically from the underside but stopping the cut just at the corner with the top flange I bent the arm up until it was aligned with the ball on the crossmember. This left a triangular gap of about 8 to 10mm at the base.I welded in a triangular piece of similar gauge mild steel as the rad arm.Car steers very nicely. I did this mod 14 years ago and have done x thousand miles in it since.