I have a Mk2 Vitesse ( Rotoflex) and previously have removed/replaced the diff using a spring lifter several times in the past, but decided to try another way last time I did the job, as described below. It may sound complex, but is easier than it sounds and saves a lot of time. However, please do not attempt this method if you are in any doubt about your abilities.
Slightly loosen,the rear wheel nuts. Jack up the car under the diff and support on axle stands under the vertical links.Then remove the rear wheels. It's very important to position the axle stands directly under the wishbone's outer trunnion bolt 'tube' in the middle of the wishbone. Lower the car very carefully onto the stands, checking the final resting position of the wishbones on the stands ( As the spring takes the weight,it will tend to flatten out slightly, so constantly check the stands position until the cars' full weight is safely supported. Check and check again that the car is properly and firmly supported.
Remove the spring fixing access plate. Find a couple of bits of wood, about 2" x 2" x 3" ( from memory) Then position these on top of the spring, directly next to the the outer edges of the spring clamp plate. You should be able to 'fiddle' these blocks into position either through the spring clamp access hole and/or reaching into the tunnel. The idea is that the wood blocks should be a snug fit between the top leaf of the spring and the underside of the spring tunnel - to stop it moving up when you remove the spring clamp plate/nuts. Ensure that the blocks are firmly wedged in position. Then position the trolley jack under the diff to just take the weight, but no more, Do not allow it to jack the car off the axle stands. You can now remove the bolts/nuts from the halfshafts and rear propshaft flange. I found that I did not need to remove the exhaust pipe,but you might have to on a GT6.
Now you can gradually loosen and remove the spring clamp nuts and clamp plate. Then using the double nut locking technique unscrew the studs from the top of the diff. You can cover the stud holes with masking tape to stop dirt ingress into the diff. Then remove the diff mounting front and rear nuts/bolts.The diff is now ready to come out, by gently lowering the trolley jack. As I was doing the job on my own I also tied the diff with some strong rope, onto the trolley jack cradle to stop it sliding about. Note that the rotoflex couplings will be pressing inwards against the halfshaft flanges, so you will need to gently lever them apart with a large screwdriver or pry bar, whilst lowering the diff out.
Replacing the diff is largely the reverse operation, but ensure that the spring centering blind hole on the diff case locates properly with the spring fixing 'peg' on the underside of the spring.Refit the diff mounting nuts/bolts.Then refit the diff studs and spring clamp plate. Remove the wood blocks. Refit the spring access plate. Replace prop and halfshaft bolts/nuts. Don't forget to torque up all bolts/nuts. Have a cup of tea.