I have been thinking of this too. I measured the pipe using a wire, you wrap it around the outside of the pipe, cut to fit, then divide its length by Pi to get the OD. The material is approx .0625 (1/16) thick so deducting twice that you get the ID. It is a little less than 2.5 in. If 2 3/8 I'd not be surprised but I can't locate my calculations but I do remember that 2.5 pipe is what is closest because I called a exhaust place that said they will make a "bypass" in 2.5 pipe.
http://www.bobsmuffler.com/ for my '87.
The old style downpipes are extinct! BTDT can't find any anywhere
I asked what about a complete new line in 3.0 in they said no problem, custom made and mandrel bent of course. I wouldn't mess with anything other than mandrel bent. Click on Portfolio and take a look at the pictures of some truck exhausts they have made. Lots of truck owners out there in the valley who want their trucks to sound like a hot rod with the Flowmaster sound
The exhaust they put on a Camaro is really sweet too. I think they to put a Mercedes picture on there
About that 2.375 flex pipe. Anything that is not perfectly smooth on the inside it will add back pressure. I would avoid it! Use a smooth pipe with flanges if necessary. A good muffler shop can use a pipe that flared larger to fit over the adjoining sections and welded in place and should hold up very well with no extra back pressure.
If I get to Bob's I plan to ask them if they can make up a few bypass pipes, for members. If anybody's interested maybe I'll put it to the top of the wish list.