Installed headers on a 93 GMC K2500 with 454, They helped a lot with that motor, moved heat from under the hood and down the exhaust. Motor ran cooler, engine could finally breath, I also had a K&N cold air kit and chip installed. The motor was a dog before these changes. The new trucks are pretty well done from the factory so not sure how much gain you would get on one.
I also had a 62, 409 dual quad SS Impala, wish I still did.