the carbs are the same, just that the one for the 5.0 has a leaner set of metering rods in it, you would probably not tell much difference running it on the 260hp. The best thing I've used to disolve the varnish, caused by old dried gas, is Berrymans B-12, but it's hard to slosh it around in that tank, got to use the tow vehicle to help with that plan. The biggest issue is pulling the pick-up tube out of the tank, to remove or clean the screen on it, the threads are prone to pull out of the tank, or off the fitting.
THANKS RED............KNEW YOU WOULD KNOW THE ANSWER.....HERE'S ONE MORE..........ANY DIFFERENCE IN A 5.0 AND A 5.7 ROCHESTER CARB....I'M THINKING IT'S THE SAME.............ALSO, I STILL HAVE A LITTLE TRASH IN THE TANK FROM SITTING A VERY LONG TIME.....WHAT'S THE BEST THING TO PUT IN THE TANK TO DISOLVE THE REMAINING TRASH.........I SEE YOU SOLD YOUR BROWN ONE AND GOT A GREEN 80' LIKE MINE....MAYBE WE SHOULD HOOK UP SOME DAY..............THANKS, FRANK (THE MAD DOG) IN NASHVILLE,TENN