Exhaust sizing

I just bought an '86 Firebird, 305. I don't really want to do too much to it, but I would like to put a Flowmaster 40 series muffler on it. Problem is that when I wanted to order the muffler from Carquest, they told me I need to get the dimensions on it, I figured they could look it up, but they couldn't.

The car is in the shop right now getting a tune up and a few other things, so I don't have access to it. Do any of you happen to know the sizing on the muffler that I'll need?

Thanks in advance.

I believe they used a 2.5 Inlet but I could be wrong but there should be no reason they can't look it up unless it's a after market system but even so they should be able to measure it then. Good luck and let us know how it turns out.
John

i have an 87 firebird and the model muffler i needed was an 80 series flowmaster muffler because of the way the in tube is on the same side as one of the out tubes because of the way it is mounted. Yours is probably the same but might be different.