If fuel economy is a big factor a cabover with the 4 banger is the way to go. I know many that have had the GMC cabovers essentially a Isuzu with a GMC logo had miles and miles of use and very little repairs. The cabs rotted off from excessive rust but the engine run like a top. Many had 400,000kms (249,000 miles) that is alot for this area and the hills the trucks see daily.
The cabovers were no big power makers so don't expect to stomp on the throttle and it push you back in the seat
I seen a LCF Ford the other day and it was running the V6 PSD sounds like a 6.0. Not sure if I would buy a LCF. One of the building suppliers bought a Sterling cabover and they seem to really like the truck.
You want a cabover that runs the 17.5 wheels the ones with the 16 inch wheels the tires are constantly overloaded.