Road race car (if that matters), so emissions / cats not needed

Current Class: stock 305 TPI, stock manifolds. 230hp/300tq

Future Class (in about 2 years): Upgrade to shorty headers (Dyno Don's?) for the 305 or swap in a stock 350 TPI w/ stock manifolds. 240hp/330tq

Searched but couldn't find it, what is Don's y-pipe diameter?

Before I goto the muffler shop, I'ld like to get some recommendations here, so I'm not suckered into buying something they just happen to have in stock.

I'ld like to keep the side exit (infront of the rear wheel). I currently have a summit bullet muffler tucked up about midway on the door, but it shows some signs of scraping the pavement. I ran without a muffler for a while, and it just became obnoxious. I was thinking of getting a muffler that will fit into the area where the cats used to be. Any recommendations?

Would I be able to use the y-pipe from Don and have the muffler in that location?

What size tubing is recommended? Is 3" overkill?

 

A 3" bullet in the stock cat location would be fine. It would fit DD's headers just fine.