Anyone with experience building tri y headers

Yes, to the extent I had to make any decisions between top end and mid range, it was always in favor of mid range or broadest torque curve.

Thread on the build here.
340 Chrysler Street Header Questions - Don Terrill’s Speed-Talk
Joining ST is easy and there is virtually no spam or selling of the e-mails.
I also spoke on the phone with Calvin. He's incredibly generous with his knowledge and experience.

I can check my emails and notes but I think we went about 9.5 to 12" with 1.5" diameter and then stepped up to 1.65" for another 10 or so inches before the first merge. It was actually pretty close to what the guy from Throttle's Performance suggested in the build thread: "1.5" for 8-10", 1.625" for 8-10" for the primary, 1.88" for 12-16" for the secondary with a 2.125" choke and 2.75" collectors will suit you very very well" We went a little smaller on choke (2") and brought the collectors up to 2.5" to match the exhaust.

I ran a couple different scenarios using pipemax. One with the current engine and cam, the other with a more stock engine. My goal was to get more torque off the corners while dropping into a more stock class. One interesting thing learned from running various combinations in Pipemax is that smaller diameters can be used with more efficient (higher VE) engines. Still haven't made the switch, but maybe in the next year.