Dumb exhaust question

So my '77 Granada with the 250 has both mufflers and the tail pipe rusted thru. I'd like to put an after market muff in place of the front one, but are muffs measured in ID or OD? (that sounds dirty!) I'm reading 1 13/16 OD at the front of the existing muff. I'd like a slip fit due to my poor welding skills.
Thanks

No, I'm not referring to the catalytic converter. This one has two muffs.

In the case of a muffler you would measure the ID and in the case of the exhaust pipe you measure the OD. IE you would need the mufler to be able to slip over the exhaust pipe. Hope that helps you

Sure does thanks a bunch