Headers vs 340 manifolds
I think that this is 100% BS
I would bet that your customers are reacting to sound and such (I'm glad they are happy!).
Then if the car is mainly a street car you have to live with all the issues of a crowded engine compartment, smashed pipes from speed bumps, etc. After years of 5 different brands of header, I scrapped headers on my street cars and went back to well chosen manifolds and a well designed exhaust system. I never looked back...
Lol, this is so typical of the arguments I have with customers when quoting a build, it's all good till we get to the headers and the customer says he's done with headers, leaks, speed bumps, road hazards, etc. lol
I tell them I dont build a big h/p engines to put in cars with stock exhaust, won't do it,
BUT - - I add, - I WILL put those hi-po ex manifolds on (pointing at a set hanging on the wall) if there's any leaks or speed bump issues with the suspension left alone . There has never been an issue on vehicle stance .
That's over a dozen builds ago, manifolds are still hanging on the wall.
These new design headers by TTI and Dougs are a vast improvement over the **** from decades ago
As to closing the exhaust, we ran "caps" that used the 3 bolts and a plate to close the header, then ran a street exhaust from a tube welded into the inboard side of the collector to the exhaust pipe ahead of the muffler.
Just unbolt the cap, and go racing.
Cheers