Conveyer is such a neat idea. I scoured the junkyard to find a good fit my 74 wagoneer, but not a single air dam was in a good shape, ended up buying one on-line that match my width, and need to figure out the best way to attach it.
The hose at the bottom is good too, the same idea is present on OEM civic crx from the 80's.
The other try was the underbelly pan on the 85 Grand wagoneer, seen here during install, I used the bungee around the block...
I used a drip pan fastened to the frame under the transmission and transfer case and covered a bit of the cat, great idea, but not so good when it is over 90F, the cat slowly warmed up the gas tank , and in the afternoon with temp in the near 100, the gas was so hot that the electric fuel pump could not draw gas due the restricted air flow under the carriage, pull over and chill out in the middle of Utah beetween Green River and Wellington... on the way back to SF....
But as you mentioned at the beginning the biggest improvement was the synthetic oil for the rear differential which netted a good 2 to 3 mpg, so from 12.5 to almost 15 and even 16 cruising on the highway 80 going East or West over a few tank refill.