Paul,
I used the dura levers for awhile. My original controls had disintegrated so that I was able to just insert the duralevers. It was a very quick and painless install. If you have to open the dash up to remove the old levers I stongly suggest that you do a better job and install the replacement levers. I do not believe you asked for my opinion but, I thought I would share it with you before I answered your questions.
My car is a 1969. I expect that the plate in question is held on the same way. There are two studs on the plate that go through the dash. There are nuts behind the dash that need to be removed. Removing the radio, glove box, ac (if you have it) and center speaker is the best way I know of to work on the heater controls. I spoke to an "expet" that told me he got in ther faster by removing the padded dash cover and cutting the metal dash underneath. It did not sound like an elegant solution and I would not reccommend it.
Best of luck,
Ted 1969 280SL