I’ll be installing a 3” exhaust - It’s a basically stock 340 but I want to go big when I do my longtubes and it will allow me to grow into the exhaust as I add more power.
Before you purchase anything, what is the power level your after? 1-5/8 header tubes efficiently work very well to a certain level. Most times, IMO, a 3 inch exhaust pipe after a 1-5/8 tube header is what I see as a miss match. I’m not saying it doesn’t work but in most cases when power goes up and starts to become a powerful engine, a larger header does better. Most times, the next size up header is a winner in power from start to finish.
Not knowing what you have in mind, my mind see’s a 3 inch exhaust connected to at least a 1-3/4 header.