Don't know about the mpg gains. I think that has more to do with gearing than anything.
As to the rest: I easily gained 25hp with headers and a 2.5" single exhaust. Used Langdon but Clifford's should yield the same results. I'm now adding a 390 4bbl and a single quad offy intake. I'm hoping for another 15-20 hp.
Adding valves and a cam... maybe another 30 hp? I'd be happy with 225hp out of my 250. Without going turbo I don't see getting much more out of it.
Now for the price:
Headers: $270
Intake and air cleaner: $300
Carb (flea-bay): $190
2.5" Exhaust: $400
Muffler: $50
Misc: $50
Total into it so far: $1260. I imagine that head work is going to set me back at least that much to get another 30 hp or so. Is it worth it? Probably not. I might stop after I get the 4bbl on it.
...or I might just add a cam and stop...
Back to the mpg: adding the exhaust did nothing there. We'll see if the Holley 8007 does anything positive or negative in the next 2 weeks or so.
You are right in line with the combo that I have. Offy/390 with stove bolt header 2.25" duels with Hpipe and glasspacks. I am guessing I am at 200 tops, I do not really know. I find it hard to believe a intake carb and exhaust will give you 50 HP over stock. All I know it the car runs great it has plenty of power for a street car. I do not see how Clifford could make these claims MAYBE wit a lump ported head and a cam with 230*duration.
