The catalyst material is different, as is the designed operating temperature. Before I had my current catalytic converter installed, my smog pump failed so I removed it and capped the line to the old catalytic converter until I could get to it. In that time the old catalytic converter overheated and cracked.
Modern catalytic converters have the catalyst matrix in a honeycomb structure. If the catalyst overheats the honeycomb can collapse and cause a blockage. The problem you will have is the fact that your engine still uses a carburetor. The fact is that carbs don't control the fuel mixture tight enough and allow the exhaust gasses to have too many unburned hydrocarbons. This leads to reduced lifespan of the catalytic converter. Having a functioning air injection system helps with longevity of the cat, but it won't last long behind a carbed engine.
More info than you ever knew existed about catalytic converters:https://en.wikipedia.org/wiki/Catalytic_converter
What you can do to get a good, cost-effective, durable cat(s) is to order (or tell the mechanic to put in) a "California-legal" converter. Those are the good ones that last, and they can be ordered in any state. The catalog of Eastern Catalytic, a reputable maker, is HERE
, and they show listings for your van. They have universal and direct fit, with and without the air injection tube. The rear direct fit cat for your van is part no. 645332.
I don't know what diameter exhaust you are running, but CLICK HERE
for a pre-filtered list of the catalytic converters offered by Summit. You can further filter by diameter and inlet size, cost, etc... I would stick to Wlker or Catco for brands.