I will be upgrading to twin aluminum 540/572"s putting out about 700HP normally aspirated. What size coolers should i run? Also currently running mercruiser 420's. Can I run the coolers for the 540/572's with these motors until I convert over. I plan on using coolers with thermostats. I need some for these motors and I really only want to buy them once.
thanks.

Going to the Hardin Marine site they list the following as the standard cooler combo used by Mercruiser:
Combination Engine Oil & Power Steering Or Transmission Cooler 620-400 Your Price $229.99

Combination Engine Oil & Power Steering Or Transmission Cooler, 2" Diameter, 19" Overall Length, 3/8" NPT P/S and 3/4" NPT Oil Connections, 1-1/4" Water Connection

All the same features as our other coolers except it is designed as a MerCruiser replacement cooler for the most common V8 engines. The only noticeable difference is the use of 3/4" NPT oil connections providing a higher volume flow capacity.

The cooler they list for a 700 hp application is longer and 3" in diameter. The 3" model has over twice as many tubes.

Could I use the bigger cooler with a 468 at about 450HP? Would it hurt anything? I do plan on using a thermostat with the cooler.

Bigger oil coolers won't hurt as long as you have an oil thermostat that allows you to cook off the condensation.

Cool thanks.

Like ben said a big cooler is great as long as you use a thermostat........3x14 bundle 1.1/4 water and 3/4 npt oil