i wanna know more bout slant sixes

I don't know what year your Jeep is, I had a 1980 CJ and have a 2002 TJ (Wrangler) now, they aren't very good on gas because they are very heavy particularly the older ones I think. Full boxed frame, front diff and transfer case adds a lot of weight and they typically have tall gears. The inline 6 they came with makes pretty good torque which you need, I don't know if a /6 would be a great idea. If an engine has to work to hard it won't get good gas mileage anyway.