not dissing your driving wilbur, but a mid 50s truck isn't mid 90s tech, and it's sure not a race car
if the chattering is so bad the cab jumps about, it's your fault for not immediately jamming in the clutch and trying a new approach .... I know that my truck can chatter sometimes, especially cold, when I don't "pay attention", if I either keep the engine almost at idle or rev it a bit more and/or slip the clutch a bit it's fine - also once well warmed up it's not likely to happen, who knows why? when the clutch is worn out it'll get looked at, could have any of the problems folks mentioned above [but milder]
it may be that the mount position has some influence [it is a 4 point mount tho with the bellhouse mounts], but I've never been into trying to improve the original design of an old truck to avoid learning to deal with it, many thousands of drivers have put many millions of miles on old trucks without modifying the technology
add: "rear transmission mount"?? I hope you mean bellhousing mounts, and don't have the car trans with a mount on it - a 5 point mounting setup could be a problem, or no bellhouse mounts so a 3 point, ditto
Bill
