Offshoreonly.com - Tailpipe rebuilding.....

So I've got a two sets of Gil/Harden 4 bolt flange style tails that have been butchered at the tips beyond the transom and are very close to the stern now; one set the inner pipe has actually been cut back inside the outer pipe by +/- 1/2" or so, dont ask me why..... At one point these were set up really well as a through hull wet tail with (from what I've been told) 6 small 1/8" holes drilled on the inner pipe @ the bottom and 8 small 1/8" holes on the inner pipe at the top @ the exit. Now I'd like to acheave a couple things here, 1st thing I'd like to do is hydro all 4 tails because I saw the slightest traces of water on a couple of the turbulators, now I know these are supposed to catch condensation but never can be too careful. I figure tig up all the ends of the tails, hydro them then if all is good I'll add 4 new sections of inner and outter tail from the furthest aft seam back including adding new 3/4" bungs for the water flow tattle tails on my stb side because thats how they were set up originally and sucessfully. So questions, what is everybody using for inner and outter tailpipe material? I believe from what I've read, it would be 4" and 4.5" 316L seamless .0625 wall SS tubing such as this Hardin Marine - Polished 4.5" 316L Stainless Steel Tubing
Another question, is 4" what builders are using inside 4.5"? Yet another question, does it matter where in relationship to the inner transom the bung gets welded into the tail? 2"-3"-5" away? Appreciate the help and suggestions, I'm by no means an exhaust guru so anything you guys could add would be great; just want to get this tested and right...
Thanks for taking a look.