I doubt that just changing the rockers would help much.

For mileage I would think that the best valvetrain improvement you could do would be to decrease the drag. I was amazed at how much power you lose to open and close valves. It is obvious when you try to turn a camshaft.

The rocker ratio has to match the cam lift and valve angles and such. It's not a simple shadetree parts swap.

I have that engine in my camper van and it seems to breath really well. It loves to rev and howls like a demon climbing hills. More revs get power better than more throttle. I would love to get better mileage. If you figure it out I would like to hear of improvements.