340 converter

70aarcuda I checked the motor its a 69 model with a mopar preformance cast crank don't know about flex plate but i watched the guy grind a weight off the old converter and spot weld it back on the new one .i had drove the car right after the mishap there was no viberation then ,trans just wouldn't change gears.after trans was built and installed i noticed it right when i cranked the car up. could the converters been built different in side so the weight was different than the old one .i'm confused as why i have a vibration now or should i say fills like something is hanging on one side of my motor as it turns .help me if you can thanks.