Slant six or v8 swap for use in a truck

I have 2 Dodge 2wheel drive trucks. A 88 D-150 with a 360 4V 727 trans, and a 64 D-100 with a 225 slant 2V and A833OD trans. I don't remember the rear ratio of the 88, but the 84 has a 3.91 SG 8 3/4. Both trucks have AC, PS, PB, cruise control.
The 88 will out pull the 84 and has more power/speed at 75 MPH pulling my race car. The 88 truck will get 9.5 mpg normal driving, or 9 mpg towing.
The 84 truck will get 13 mpg towing, never checked the milage not towing. Myself, I would rather have the fuel milage. I don't need the additional power of the 360 engine.
Not positive what I would do if modifying one of those, today. I might consider building a 318 engine for economy, and using a A500 or A518 lockup trans.
Another thing to consider, is adding a turbo to the slant and limiting boost to around 5-7 lbs.