Flexplate 400ci or 350ci?

[roadrunner-europe]

Good morning,

i´ve got a problem with my ´77 pick up.
First i have to say, the engine never runs well.
According to the VIN there must be a 400ci inside

CKR247F511075

Whenever something´s broken down, i ordered spare parts by this number.
Till now this runs without problems.

But now my flexplate is broken and i ordered a newone by this number however.
As i received the new, it was different. There is a balancing weight on it. After a call with the dealer, he told me that the 400ci-motor is balanced on the outside via the flexplate. So i think by myself, that this could be my problems with the engine. So i mount the new and start the engine.
Immediately the truck began to shake.
The balancer on the frontside of the engine is a 8", typically for the 400ci.
But with the flaxplate now, i don´t know further.
Is there a 350ci or a 400ci engine in?

Thanks for helping
Greetings
Chris