necking down after collecter?

Oh, I a case of dyslexia so bad I mix up words with words, letters with words, letters with letters, and numbers with numbers that have no similarity what so ever. The exhaust extensions inlet is 3" it tapers down rapidly to 2". Sorry.

Are you talking about a collector? The piece bolted (or welded) to the end of the headers. These can be exchanged for larger and in your case should be.