Hey, I did this today on my '93 T bar.
Basically I trolley jacked under the centre of the rear subframe, then placed 2 axle stands just to the left and right of the trolley jack. Lowered the subframe down on to them (with car chocked ad in first gear - on the flat) Car very stable and gives more than enough room to get under it.
As a new owner not sure entirely how much undertray there is or of you need to take any of it off but it looked like mine was all there and I didnt have to.
So .... get on your back, undo the 14mm alternator pivot bolt (as per excellent diagram in above post), then undo the 12mm adjuster lock bolt from above. Slightly tight in engine bay but more than enough room to get a spanner / socket in there. Then you can just adjust the tension with the adjuster screw (again, 12mm bolt). Dont go mad on it, Adjust it a bit, get back under car and check tension on the belt, then get back out and adjust it some more as necessary.
Useful advice given to me yesterday on these forums .. 'dont overtighten it .. you'll goose the bearings'.
Self explanatory statement I think.
Once you've got it as you want it, retighten the adjuster screw lock bolt then get back on your back and tighten up the 14mm pivot bolt. ... job done.
N.B. I was more than happy my car was stable on the axle stands but I still didnt start it just in case and I would suggest you dont either.
To be honest, another way to do it might be to drive the rear wheels up on to a couple of bricks (maybe a bit of wood first to lessen the step). That might well give you the clearance you need to get under the car.