I am dealing with the same problem right now on my '51 1/2T. See the attached pictures to see if we are talking about the same thing.
I will measure the clearance between the plates tonight and post tomorrow. The replacement rubbers shown are from Chevy Duty. They appear to be too thick (by about double). I guess they will put the mount too high for the mounting holes on the back end of the transmission. Since there were no instructions and the guy behind the counter didn't have a clue I guess you are to cut the rivets off and replace with bolts.
The original mount appears to have large holes in the plate that had rubber bushings.
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http://www.whcinc.com/images/TransMt5.JPG[/img]
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http://www.whcinc.com/images/TransMt2.JPG[/img]
