Recently had the manifold pipe replaced on my 84 244 (I bought it from FCPGroton - had local mechanic install). Since then there is now a loud rumble and vibration when I accelerate (3000 RPM+), which goes away as soon as the transmission shifts and the RPM's drop back down. The car feels less powerful than before also. Once I've accelerated hard a few times causing the noise and vibration (with warm engine), the car seems to have more power again in normal driving.

The mechanic mentioned having a tough time removing the cat-to-manifold bolts.
Could this be a clogged cat or maybe something entirely unrelated?
Any advice humbly appreciated!