I have E350 cargo van that is sputtering while accelerating and lose power. Any suggestions?

 

Possible fuel pump, or catalytic converter clogged up

 

Engine, mileage?

 

The engine is 5.8l V8 and the mileage is about 200K.
The car is purring at idle and is running good at cruising speed but if you have to slow down for any reason (bad traffic mostly) and thentry to go at the traffic speed againthe engine will "dieseling" for a whilebefore getting to the previous speed.
I change the spark plugs and fuel filter. I was thinking to change wires and capdistribuitor.I was alsohopping that therewill be something easy then converter or fuel pump.
How about oxigen sensor, orTPS? these can cause this problem.
Thanks guys!!

 

For a 1994 E350 5.8l engine if Iwant to change the CAT, do I need to weld any parts?