You have a 2 piece driveshaft in that car. Do yourself a favor and lubricate every zerk under that car especially the center driveshaft bearing. It is constantly forgotten because no GM car has come with one since the end of 1970 model year production. Do trust jiffy lube to take care of this for you they don't know it's there. If you notice an overall vibration in the whole car it's a bad center driveshaft bearing. It was eliminated on 65 and up B Bodies but big car C and D bodies had it until 1970.
Check to make sure rodents haven't moved into the exhaust system. That could be a cause of rough running and no power. A friend found a rat had a litter in the resonator of a 64 Cadillac and had these same problems.
Check your engine code to make sure it's the original 283. Many later gaskets and such won't work on that engine 65 and newer engines have many differences. You need to know this before ordering parts.
Good luck with it. I know it's a cool ride because it's as old as me!
