Replace the 318 with a Slant

So I am thinking of replacing the 318 in my 3/4 Dodge truck with my 225 2brrl that I had built a few years ago.

Reasons are mostly mpg the 318 is runs great and has no problems.

How do these trucks do with the slants? Should I still get about 28 mpg or will it drop to 18?

Is it worth it to swap the motors?

I am not driving the car with the slant because it needs the whole suspension and brakes done.