vtchevy

I have a '98 chevy 2500 silverado that I just put a re-manufactured 350 in and had a new, complete exhaust system installed, and STILL I have a power problem "off the line" with it!! I was told the cats were clogged, and so redid the system, as stated. We figured this was it because once the exhaust was disconnected from the motor the truck went like a raped ape. And now ... it isn't as bad as it was, but it still is sluggish as hell going from the get-go. If anyone has any ideas about why, I'm really open to them. Thanks you all.