Requesting cooling/radiator/fan help for a slant six

A high-performance engine will not generate more heat in daily driving if it is as efficient as the standard engine. That is basic thermodynamics. The reason many tuners have over-heat issues is that they get 8 mpg, with the extra energy going into heat load. Max HP is due to maintaining torque at high rpm, and mainly from how well you can minimize pressure drops if the gas flows into and out of the engines. Improving that doesn't necessarily ruin low rpm operation.

I understand that 3 cores gives minimal improvement over 2 cores. I suspect the M-B clutch fan with the thinner radiator and shroud would cool adequately, and give less power rob than a fixed fan. Of course, what you need depends on conditions. The OP lives in SW Washington, near the Pacific. Somebody who regularly drives out of the LA Basin (steep 4000 ft climb at 70 mph) on 115 F days certainly needs better cooling, and new vehicles barely meet that. I understand manufacturers test their new designs there. With a little age, most vehicles no longer suffice which is why you see one (often fairly newish) on the side w/ hood up every 1/2 mi on summer days, along I-5 N (Tejon Pass) and I-15 N (Cajon Pass).