I would tell your "mechanic" to take a few lessons from pros and not be scared to ask other exhaust specialists the stuff they really don't know anything about. I mean, I hope they aren't installing the cats if this is the ignorance they are proclaiming to you - I'm picturing them installing the cats with a propane torch and pipe solder wondering why the pipe isn't getting hot enough...

Basically, get an "SUV" or "Truck" universal cat (or the Isuzu ones listed above by VCrrossfan) and you're fine, it'll be sized robust and larger pipe ends.

If you try and stick on a "car" universal cat with a 1.5" or smaller pipe end, then I'd be concerned...

Good luck man, and keep up the patience.

NOTE: high flow cats will increase exhaust volume, but probably not all that much with that monster stock Isuzu muffler...