1976 three on the tree question

I drove a 79 B200 shorty van with a 318/833OD and 355 gears. I got upwards of 25 mpg when empty.
Yes I have also had a lot of pickups with the od.
One I had was with a slant and it was gutless, I even tried putting 4.56 gears in it, still not worth a crap.
Put the 3.55 gears back in, and put a NP435 trans in it, way better truck after the trans change.