Yes stack height should be around 38mm. I little less doesn't hurt too much but more can cause problems.
There is quite a bit about pushrods on this thread if you're prepared to wade through it as it bifurcates into to two subjects. See posts #47 and #57 on page four.
http://www.ukmonster.co.uk/monster/s...g+oberon+slave
Your pushrod should be 325 mm long and should be used with the 10mm spacer.
You will have "anti-rotation slots" in your slave and so you could convert to an anti-rotation pushrod which is 335mm long and would be used without the spacer. Part No. 11721122A.
The clutch end of the pushrod needs to be all in good order too, so the top hat that the pushrod sits in needs to be clean and lubed with a little grease. The thrust bearing needs to be good too. They're very cheap and should be replaced routinely.
You also need to observe that the pressure plate is correctly installed with the arrowed (a big triangle actually) spring hole, positioned over the spring post with the slot. (it looks like a screwdriver slot.)
You may have a wear in the outer drum which is making the plates stick in the grooves? this will have a serrated appearance to the drum slots which the plate tangs engage in, you might also have burrs on the plate tangs? There's not a lot you can do about that except replace the drum and plates as filing the serrations and burrs just accelerates the wear.
Back to basics: it's a very good idea before you reassemble the clutch, to wash all the plates in clutch and brake cleaner to remove oily residues and dust. This alone can often transform a clutch.
Hope that helps.. Let us know how you get on.