The easiest way to improve power on a 403 is by swapping the cylinder heads and getting a well-matched camshaft. Look for a set of #6 small-block heads from a '72 350, then have a machine shop install new 2.00" intake and 1.625" exhaust valves with a 3 angle valve job. Contact Crane or Comp cams for a cam recommendation and get matching valvesprings. These mods will increase your cylinder pressure (compression) and dramatically increase your engine's overall combustion efficiency (which = more power from the same size engine). Last, get a good dual-plane intake manifold like the Edelbrock Performer and small-tube headers. You'll have more power than you can use legally, really.
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