Should I rebuild this engine?

I drive my dads 2004 Ford f-150. It was bought brand new in in 2004. It has 336,000 miles on it. Only thing that has failed was the alternator, that was at about 246,000 miles. The alternator is the the only thing that has ever been replaced beside the spark plugs, oil and all the filters.

I just want to know if we should rebuild it because its not normal for a gasoline engine to get that many miles. I don't want it catastrophically failing on me internally

Leave it alone. If it runs good, and has lasted this long, something was done right on it's construction.

Drive it until it has real problems. You can't fix something that isn't broken, but you can break something by trying to fix it.

If you take care of a gas engine it can easily get more miles than that. So if it sounds alright leave it alone.

Is it a V8 or V6? The V8s Ford put in their F-150s were either 4.6s, or 5.4s. Both are basically the same design, the 5.4 just has a longer piston stroke for more displacement and more torque. Both are extremely reliable, and will last dang near forever if you take care of them. 400,000 miles out of 4.6s in Ford taxis and limos is quite common. If it's running good, don't worry about the mileage.

Now a answer you can use. With a engine with that kind of mileage you would have piston cylinder wall wear that would require you to bore the cylinders out, that in its self is expensive and if you are lucky enough to have a crankshaft that has not worn much and can get away with only polishing it up and using oversize bearings that would be nice, but if not it is another trip to the machine shop to turn that, again costly. I know it would be fun to do the rebuild but in reality the cost would be high. Now if you want to get a Jasper rebuilt that would be a good deal better. I'd take a look around the truck to see if it will be able to last, rust seems to like F-150's.