Balance, compression on low side, no need for head work other than better guides seals and a valve job, slow bleed lifters, valve springs with appropriate pressure, rigid pushrods, rocker lockers, a cam with mild ramps, forged pistons that fit snug (4032 alloy) with Line2line coating, cases bored and full compliment spigots on the barrels. It is a 117" motor, it is big. It does not need cutting edge anything to be fun to ride. At 1.05-1.1hp per cubic inch (will require mild headwork to get there) built right and tuned properly with a decent pipe it can be very comfortable and trouble free. Move up and the care and feeding increases, octane requirement, noise, heat, and vibration.
Contrarian I know. Very few realize what they ask for they get, a high performance engine. Now they realize they gave up some to get there besides money. Next they start the cash throwing to chase the problems. Better to start with as clean of a sheet of paper as possible and look at what needs to be changed and what doesn't. Many of the HD OEM parts are fine, others not. Set a realistic goal. This will get you a cool running quiet twin cam, as smooth as you can get but it still is an uneven fire Vtwin