I swaped out the intake on my 65 with a weiand excelerator and put on a holley 670 street avenger.Couldnt get the holley tuned right so I put my edelbrock 650 back on.Found out the reason it wasnt running right was because the vacum cap I put on the manifold was split.Now when I take off I get a bad vibration(it feels like it comes from the rear) at about 15-20 then really bad at 50-60.It smooths out at 60mph and doesnt return unless you get down to 50ish again.The manifold gaskets leaked when I first put them on so I removed them and replace them then used a different silicon.I changed out my plugs and it didnt help.Put my car up on the lift at work and check to see whats going on.One u-joint was bad so I replaced both of them.Still didnt help at all.I checked the tranny mount and its still good.The motor mounts are brand new.The weight isnt nocked off of the driveshaft or anything.It only vibrated really bad at 50 when your giving it gas.And when I first take off once its cooled down itll shif itself into 2nd gear(green dot C4).I think do to the vibration untill 20 mphI rotated the rear pinion to see how much play there is and it does have some play in it but it doesnt move up or down at all.I didnt have this problem before I swaped intkaes and carbs.And it did the same vibration with the holley and the edelbrock.