Chevrolet Repair: performance bolt ons, electric water pump, horsepower rating

Question
i have a 68 chev impala,307,2speed powerglide,and stock differential .the horsepower rating in my manual is 210 hp.it also has 9:1 compression.what kind of extra horsepower could i expect from the addition of headers,4barrel carb,intake manifold,and perhaps freeing up horsepower with a electric water pump and a electric fan.is there more parts i could add or replace that i did not mention.thank you.

Answer
Without going into the engine as far as cam, heads; Small tube headers, 1 1/2" to 1 5/8" NO larger! Then 2 1/4" exhaust pipes back. Edlebrock #2101 intake, with a well tuned Q Jet carb. Curve kit in the distributor, with a vacuum advance unit with at least 16 degrees, hooked up to "ported" vacuum,(0 vacuum at idle). Comp Cams "Magnum" rockers would be a benefit. I'd stick with a fan clutch with the proper shroud, as electrics really cause more problems than you know!! If done correctly, a 20-40 H.P. gain to be had, as, without a head change, the 307 heads just can't flow enough!! A cam change with different heads would be a great benefit for the 307! Hope this helps.