Small block or big block in a truck?

I have had alot of Dodge trucks, slants, small block and big block.
The big block will move the truck much better, but will be thirsty, my last 440 powered truck got 4 mpg.
Of the slant truck I had, I remember towing my Jeep CJ thru the mountains, and it was painfully slow, I got pasted by a semi, up hill on a two lane mountain road.
I told my wife I need to un-hook the jeep and pull the truck, the jeep had a SBM and a 435 4-speed.