Biggest enemy is allowing to overheat- good radiator, proper water, functioning thermostat, good water pump all critical. General durability is excellant- 150K miles untouched except change oil and adjust valves not unheard of. But, over heat once and expect a crack, ususally from the valve seat to a cylinder and into the water jacket then internal water leaks into a cylinder and loss of power and every related problem. Sleeving and valve seats not always successful so be aware. The basic engine was used for years as an industrial power plant, fork lifts, military gear, Kaiser Jeep Vehicles etc. Crank appears bullet proof, rest of block sound. Really not a lot to do to get more power out- even balanced and ported maybe 5-10% bump, hotter camshaft rare as hens teeth, some multi carb manifolds- Edmunds, possibly Ellis from truck applications, but members suggest some added power but big mileage loss. Fairly new as well but i read and listen to other members and their years of knowledge. Oh exhaust manifold cracking- in part due to overtorque of mounting bolts also appears to be a topic of note you may want to explore.
SK