Any modern engines or engine builds for modern results?

The efficiency Magnums have is mostly in the heads, manifolding, and accessory drives. The roller lifters don't do much because they're heavier and the pushrod angle is worse. So once you take an LA 360 and change to those Magnum-based heads, replace the pistons with modern hypers or forged, stick in a retro hydraulic roller, and run headers - there's no real difference. Price-wise the differences are the cost differential of the lifters and the cam you chose to run, and the valvetrain for the Magnum setup. The last hydraulic roller I did I use stock lifters (roller LA block with LA heads). Much less $$ than any performance name brand version lifter and no performance drop.
The VAST majority of bad gas mileage reports are due to poor tuning ability. Mechanically it's not hard to make them efficient and good power is a side effect of that, but some people just don't have the experience or hands-on tuning talent. It takes very little to get mileage to drop 25-50%. Tire pressure and alignment alone can drop it 15-20% and that's on a modern car with all the electronic controls, OD, and weight savings efforts.