You might try checking your cat converter to see if it's clogged. It happens on them. I have seen a number of them cause a lot of black smoke and power loss etc.
If you take it loose at the downpipe end you can look in the converter and see if there is a soot build up. Also drive it with the converter loose. That will really tell you if it;s clogged.
96 6.5 ext cab C2500 short bed, 2 IP rep. under warranty. Came with Dual Thermostats & HO water pump,Tweaked fan clutch engages @200, Kennedy exhaust, Heath Turbo Boost, Heath ECM
John Deere Tractor (It likes cetane improvers also)diesel65j@netscape.net (not always monitored)