Want to upgrade the exhaust on my Slant ...size matters?

Hey MoPros, I have 66 Dart with the old Slant. Has the stock 1 3/4 exhaust from mani to tail.

I cut out the old muffler and clamped in a new Dynomax 17514. It as 2in in/outs.

I have swapped in the Super Six setup, etc... which suggests I should upsize exhaust to 2 1/4.

Is that correct? And is there any issue with me going 2 1/4 to my new 2in muffler? then 2 1/4 out of the muffler?

What makes the most sense? Ideally reusing my new muffler. Thanks guys.