Forgetting the bushes for a moment does anyone know the thread/nut size of the 2 bolts that hold the spring pan in place. I thought the outside one was M14 fine 1.5 pitch inner one M12 1.25, but the flanged nuts I have that are supposedly those sizes don't fit. Off to the main stealer to get the right ones I suppose. I thought that Lancia were the only ones to invent their own Metric sizing...
Attached is a picture of a check I did this morning out of curiosity on my 2 litre GTV hulk suspension bolts (fortunately dismantled but hiding skillfully inside the boot and it was blinking cold outside!!)
Inner bolt (springpan to rear frame) has an m12x1.25 thread and is 90mm long.
Outer bolt (springpan to hub carrier) has an m14x1.5 thread and is 170mm long.
Note the threaded inbuilt flange on the nut to spread the load onto the face of the 3mm x 50mm dia. SAFETY washer.
If you are replacing the nut it would be good to fit an extra 3mm x 30mm dia washer behind it to simulate the original. Don't forget that the original flanged nut has about 17mm length of full thread.
So the bolts look to be the familiar bog standard cad plated steel issue, (probably equivalent of EN8 or 16 in 'old money' measurements) the inner one is of small diameter 'cos it's in double shear and the outer larger because the dogleg load is overhung.
So it looks as if the nuts you described should have fitted!
Another point about the suspension harshness - When you had work done on the suspension the garage should have tightened up the bolts through the rubber bushes when the weight of the car was on the suspension. If this was done with the suspension unladen then the bushes could be acting as a significant spring assister (remember the Hesketh F1 and Moulton bike suspensions?)
My GTV6 uses the standard (original) springs and I changed the aftermarket 'uprated' dampers for used original spec units which restored the handling and comfort levels and no-one's complained about the ride comfort. (too terrified by the driving to notice perhaps)