Roger. Sorryabout my answer.
Yes i have fitted a spacer on top of a differential. On our 1500 spitfire as you know this has a swing spring as has the GT6mk3 none Roto.
I took out the rear spring leaving the 2 Studs rear top then then drop the block on top .Now for extra room to move the spring with SW Hausing
support the diff, and lower the diff just 3cm there is enough room its a bit tight but works.The exhaust back box must also be removed. I found it difficult to put the
studs in to locate the holes so leaving 2 studs in place made fitting easy. Also check the length of your studs I had to put longer ones in "lower the Diff, more room"
Don't tighten the spring down on top of Diff,leave nuts enough to hold in place. Replace Diff, using a spring lifter place the outer bolts in place then the drive shafts ,know tighten down the nuts on the Studs.
With the wheels back on put a support under the wheels the weight of the car will help. spring end bolts can be tighten up.
I hope this helps you. The thing was it didn't Work very well got a new spring from Owen's.
Chris