Oh crap..... Internally balanced engine with an external balance flex plate?!

If the engine was balanced INCLUDING the balancer and any flexplate - it is not "internal balance". That's because chances are there was some weight that was in one of those parts that the balance job used to help compensate the internal mass. If that amount was small enough - no big deal to swap parts - like if an internal flexplate was used. But - if the flexplate you gave them was not nuetral, it is not internal balanced.
that all being the caes - if the car vibrated at anything beyond idle then you have a mismatch. BTW - 1400 is NOT and idle - you're already off the carb's idle circuits. Size has nothing to so with shake.