exhaust pipe diameter for a mildly modified 340

ir3333 said
I thought a street engine needed some back pressure to keep lo rpm exhaust velocity up for low speed power?
Isn't that why street driven engines benefit from a small cfm carb and why the factory reduced the tail pipes
to 1 7/8" from 2 1/4"? This wouldn't apply for an open header race engine at 6000 rpm.
....but the question is how much power might be lost going from 2 1/4" to 2" exhaust? I'm wondering if there
may even be an improvement below 3500 rpm?

You need to search and watch David Vizard Powertec 10 videos. One is about zero loss street legal exhaust. Another delves into equal length headers.
Ben Alamida also has a video wherein he states backpressure requirement is a big lie.
Get a copy of Vizard's How to Build Power book, that will clear up a bunch of issues.