headers vs manifolds on a 318 2bbl difference ????

Zero experience. Most likely will stay that way my remainder few years of my "working on car life" ... LOL
Okay, yeah the time slip and Dyno numbers can go hand and hand as long as the operator chooses to optimize the car to the engine. Anything can make a car not perform it's best, tune, fuel, suspension, gearing all that and usually doing it all at once is best but yeah a lot of people do things here and there but sometimes forget that every action causes another reaction