[aardvarcus]

Get an old Isuzu NPR cab over box truck with the 3 liter 4 cylinder diesel. [Edited to remove sentence incorrectly stating the capacity of an NPR Box truck.] I have seen the late 80s early 90s ones within your budget in working condition. Remove the box off of the back (sell it as a storage box to recoup some investment), and shorten the frame to “pickup” length. From the cab back build a fairing that tapers in all dimensions simultaneously down to the back bottom to match the template, making an integrated storage box. That would get you pretty good right off the bat. From there, your next steps would be weight loss, changing rear axles (narrower, single tire), gearing, etcetera. Do a little research on people who have transplanted those engines into non-aerodynamic heavy vehicles with good results (stock land rovers at 26 mpg).