Depending on how it is built, the L8000 is probably more truck. The 7.8L Ford Brazilian diesel was a decent engine, but was discontinued 25 years ago. Parts could be tough. If it’s a HD 10 speed with a hi/lo built into the transmission (as opposed to a 2 speed rear), it’s a “big box” transmission. It should have counter rotating shafts and bigger bearings. The other truck with a 5 speed is a “small box” transmission. Lighter duty.
They’re about a toss up, but I’d keep looking. Knowing your budget and job description, you might be better off looking for a highway truck. 120 miles isn’t exactly a short trip, let’s say 2.5 hours. Be a lot nicer on an air ride seat than a bench seat.
Me? I’d take an Allison auto. They are really nice and they will flat out scoot compared to shifting.
You should be able to find a custom haulers truck with higher miles, but they’re easy highway miles.
Not trying to discourage you, either of those trucks would do the job, but you have to watch what you’re buying, or you’ll end up back where you started, looking for the correct truck. You’ll need air brake endorsement with either of those.