Only one thing needs said about the 15BFT. MEGACRUISER.
Trucks use long stroke 4 cylinder engines because they have great torque at low rpm and the best fuel economy. Passenger vehicles use 6 cylinder engines because they're smoother, quieter and spin faster.
Your currently tweaked 12HT should be around 200hp already. That should be ample power for a vehicle like that to safely cruise at highway speed and not waste all day going up hills.
Trucks use long stroke 4 cylinder engines because they have great torque at low rpm and the best fuel economy. Passenger vehicles use 6 cylinder engines because they're smoother, quieter and spin faster.
Your currently tweaked 12HT should be around 200hp already. That should be ample power for a vehicle like that to safely cruise at highway speed and not waste all day going up hills.