Inlet/outlet diameter

[OxProbeGT2]

I am ordering a new catalutic converter and I was wondering if anyone knows what the diameter of the stock converter's inlet/outlet is. I am about to order the Hedman converter but there are two: a 2.5" and a 3". Which one will go onto the car the easiest?

Also, any opinions on the direct fit MAREMONT?

Thanks in advance.