95 Firebird Formula Exhaust Manifold

I have a 1995 Firebird Formula, V8, 5.7L, 6 speed, 110,000 kms. Just had the right side exhaust manifold replaced because it was 'broke" I was told.

Now my car runs worse than it did before! The entire car shakes, there's a low roar/rumbling sound throughout which causes all kinds of buzzing and rattles to dash, etc. and it feels like it's running in "bow low" gear....like a tank. And it just didn't have the same power as before. The engine seems to be running fine, it's not stalling or anything like that. I can hear a noise tho, faint ticking & sound of air. Hard to explain.

Commuter error code only showed faulty ERG value so replaced that as well as a sensor.(don't know what sensor tho). Has a bit more power but still shakes.

My Mechanic says he doesn't know what it could be. To install the manifold the motor mounts were taken off and motor had to be propped up. (pretty tight quarters under the hood!)

I think something got damaged when the motor was propped up. Mechanic says no.

Then he tells me he couldn't get the "right size manifold" for my car anymore, and that the one he got has the same size bolt pattern but it's also "only" 16th of an inch difference in size(smaller) but he said that shouldn't make any difference. (I'm not a mechanic and "just" a girl :-)) so I'm guessing this means a difference in size where the exhaust flows thru.

So I think that between perhaps something getting damaged when motor was propped up AND/OR the fact that the manifold is 16th inch smaller is why I having these troubles.

Seems to me that even 16th inch size difference on a part for a high performance car is the wrong thing to do.

Can anyone explain why it is doing this?

Thanks!