68 GTS Big Block Exhaust Manifolds

Just bought a 68 GTS Big Block. It has , I am told, a set of incorrect exhaust manifolds on it currently. In the market for a factory correct set of exhaust manifolds......here's my question....am told my manifolds are not the factory stock manifolds....may be off a truck.

How critical is it to have to proper factory manifolds? Are there performance issues to be concerned about?

Is this an issue "if it fits then they are Ok or do I need the correct manifolds?

Thanks