Well, I would take a few degrees timing out of it. Actually since you run a carb and I'm assuming a non electronic distributor, I would get a curve kit that would allow for more initial advance (to keep torque), but wouldn't advance much at higher RPM's (to keep max power low). Or if you do run an electronic distributor, a timing retard for the after race dyno pull could be handy .

Fatten up the carb a bit. This will let the motor run cooler for reliability but also lose a bit of power.

Run a thicker oil.

Play around with cam timing so the thing makes alot of power at a lower RPM and gear accordingly.

Run a manual fan on it. Will kill probably close to 10hp, and let the motor run cooler all while being simpler.

But you're looking at this all wrong. You don't need to make less HP, just less HP while they are dynoing it

You could run a huge ass alternator (more HP to turn) and somehow create a huge load on a switch right before they throw it on the dyno. That would rob a couple more HP.

As I mentioned before, use an ignition system with a retard and a hidden switch.

Hell, you can safely run up to about 10% nitromethane and make an extra 10%. Do they let you refuel before the dyno?

Maybe have a line loc that you could apply some brake pressure so the wheels drag during the dyno run?