I got my bush clamps today (from brickwerks) so did a trial fit.
I drilled the brake pipe clip hole out to 10.5mm for a start but think it will need to be 15 or maybe 16mm to take the drop arm tube and allow it to move around slightly as the angle of the drop link changes.
I'm feeling happier about the stresses on the arm now, if you reach inside the arm you can feel a strengthening piece welded into the top of the arm, it stops just short of the hole and leaves enough room for the lower drop arm bush that you're going to fit in there.
The area around the hole is slightly raised so I think I'll weld on an extra drop link dished washer, upside down, as a strengthener around the hole.
The arm is 3.5mm thick and the washer 1.5mm giving a total of 5mm. sounds about right to me.
I may need to shorten the tube slightly to make sure the bushes grip the arm.
On the chassis bush clamp side.... it looks like the back edge of the clamp needs to be level with or forward of the slotted hole in the top of the chassis rail in order to keep the top of the drop link clear of the suspension tower.
More to follow.
Thanks for the encouragement Ian, Glad I'm not alone doing this.
'86 1.9 DG, 4 spd, tintop, camper conversion.
Split case club member.