Into the weeds engine design for fuel efficiency discussion.

For a Dodge D150 and towing....

I think I'd find a wrecked Dodge Ram 1500 with a Magnum 5.9 and steal all the wiring and computer too.

Bore it 0.040" and 4" stroke it to 410 and use some UTAwsom parts.

A Magnum "Voodoo" style fast rate lift 262 to 268 Degree cam advanced 4 to 8 degrees.

Deep sump Kevco 8 quart oil pan with a scraper, run 6 quarts. Stock volume oil pump with a Kevco Oil pump cover. Use the 3/4 groove 360 Main bearings. Open up the main bearing feed holes to the oval hole in the block.

The Mag engines are already larger Oil passage like the Sanborn mod done to LA blocks

Then use a HP Tuner to flash the computer.

A prepped A-500 trans and a set of 3.21:1 gears

This shoud give about 415 HP and over 500 Lb/Ft torque from 1500 to 5000 RPM