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Personally, if I couldn't have my current set of headers (no longer made) I would go with 1-7/8. My tuner has not seen major loss of low end and only benefit on the high end.
I recently talked to ARH and Kooks, as well as my tuner about the size of primaries. All said 1-3/4 would work with my planned mods but they haven't seen any loss from going with 1-7/8.
1-7/8 would be good. I think the cam is good enough to make use of the wider primaries. Might pick up a lot more horse than torque, but I think its worth it.
1 3/4 will work perfect in most cases. 1 7/8 are great if you plan on putting down some big power one day. You should be fine both ways in your current form. It's really your decision.
On the AR 1 7/8 I bought, I actually measured them when I got them, as they looked a little off. All eight tubes were 1 7/8 Inside diameter, like 2.15 outside diameter.
From: Wylie TX --> Less is More, except under the hood !
LG's tested both the 1 3/4 and 1 7/8 and said on a stock cube 346 the 1 3/4 pulled more torque.
I spoke with Anthony at LG's a year ago about that.
I'm running 1 3/4 and very happy with my results.
On a bigger cube motor you would benefit from the 1 7/8.