is the clifford header really worth it and how much of a difference does it make

Cost affective? who knows.I'm running a set of their shortys and I was only after a true dual exhaust so no numbers from me for torque,et,mpg etc. MisterTwister is building some nice looking headers that might work for you and I believe less money than Clifford.And don't forget Dutra Duals if you're after twice pipes,carb heat option and last forever material.

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83 B-150 slant 6,4 speed. 79 B-300 360 pathfinder 4x4. 74 W-300 318 4x4 git-r-done 80 B-100 sl6,4speed

Not enough info here...
Free-flowing exhaust does make a Slant run better but there are lot's of ways to get better exhaust flow, tube steel header(s) is one of the ways.
What type of car and driving? Street or race?
DD