Without knowing the heads and thinking they’re going to somewhat stock magnums heads, a .030 over 318 with a four inch crank here is PLENTY enough for towing and whatever else you’re intending on doing with it. Hopefully it’s got a Power-lock Dana rear under it. Club cab W-150 with 3.90 gear and 35 inch tires. 391 inch stroked 318 backed an NV4500 gas transmission. This the 11.4:1 Eddy head/comp 274XE variant, but a 9:1 junk gas build with a mild torquey cam would blow the rear tires away at will.