I tend to disagree here. The SM420 is an old transmission, and just bolting up another one might come with its own host of problems. Mine too likes to grind if I shift too fast. The synchros are obviously worn, but otherwise no other problems that I know of. I'm sure most of the old trannies out there have similar problems.
The only way to guarantee "new" operation is to rebuild either your original tranny, or go get a used one and go through it. Unless grinding gears do not bother you...they bother me though.
