What kind of GPS speeds are you getting out of your 260 with the 500 efi? I'd like to make a trip to Pittsburgh area this summer....

fastest i have got in the river was about 78 gps, it likes a lil chop and that can be hard to come by on the river.

600hp in the 260 (24ft + swim platform) you should be in the low 80s depending on load and water conditions. I knew a guy one time that was telling me about a similar problem and it turned out his GPS was set on Knots.........lol

Dont feel to bad. Same here, just had a 547CI put together n/a 630hp on dyno. Its in a 25' 89 Baja, I can only get 72mph on gps 5600rpm 24" labbed 4 blade.
Then again it is a Baja.
I figured i would see a easy 75mph

I don't think you are putting out 500hp let alone 600. If you were you would be spinning a bigger prop. My almost stock 502 mag (headers) spins a labbed 26 four blade hydro. This is in a 280 Velocity. Depending on conditions I can hit anywhere between 70 & 72 mph.

I got a 02 260
Supercharger and lots motor work spinning a 30p CEP or B1 at just under 5300.

Got to say your down