Those are nice. I just put a set of them on my 68 fish. 3" all the way from the collectors through the X pipe to the flowmaster 40s. A tad loud and may try to get the muffler shop to put a set of 3" x 12" Dynomax bullets on the straight sections of the exhaust between the collectors and the mufflers.
Look into the TTI X pipe system or order the X pipe kit from Jegs and get your muffler shop to copy the TTI system with it. I saved over $100 by having my local shop do mine over the cost of the TTI.