My Burb doesn't seem to be shifting when it should. Engine (350) has good compression all around, new plugs, wires, etc. It revs up to about 4000 and hovers there too long before shifting to next gear (under pretty hard acceleration). I don't know much about transmissions, especially what tells it to shift. Side note, there is something rattling inside the catalitic converter. At 145,000 miles, I'm sure it wouldn't be a bad idea to replace it, but if it is blocked some, could it cause my shifting problem? Any advice would be welcomed.

The core of your catilatic converter has broken loose. It should be replaced, and that will definatly cause your engine to rev really high before shifting because its having a hard time breating.