The 2500 RPM was the prop speed. That would be 5000 RPM on the engine.
I accounted for the gear reduction. So a 2" pitch increase would give
you a 4.7 MPH increase. I know it is not a 100% efficient. And whether
or not the extra 15 HP can turn a 2" bigger prop 2500 RPM I do not
know. But if do several modifications that give you 15HP here, 5HP here
and 10HP there it all adds up. You can do headers, Fuel injection,
upgrade the ignition, camshaft, computer etc... The 4.3L motor is very
a versatile motor. I personally have seen (out of vehicle on a dyno) a
10 hp increase just by adding a spacer (a $100 item
http://www.jetchip.com/TBI/chevFSTBI.cfm) under a TBI unit on 4.3L
motor. I would put one on my Blazer but I would not be able to close
the hood. I have done a lot of work on the 4.3 in land vehicle
applications. Limited, but expanding, in boating applications.

Paul