360 engine/ headers?

Looking for some opinions from folks who have more experience.
I've got a Dodge 5 ton Winnebago with a 360 engine & higher cams cuz it's in a class C winnebago. Still have the original Holley 2bbl carb in it. Just put a new crate engine in, and got talked into putting headers on instead of the stock exh mnflds that I still have on hand. I've lost a bit more gas mileage here and I'm wondering if the headers were a mistake... Does the 360 want/need that bit of backpressure from the stock exh manifolds? Any ideas?