Agree with all the comments so far - anything better than 15 mpg will require a diesel or EFI V8 swap.
Full disclosure - I have daily driven my 60 for the last 4.5 years with an 5.3 LS / NV4500 swap. 33" BFG KO2s, stock 3.70 gear ratios, lightly loaded aside from bumpers / winch / sliders, and...