Hi, Welcome to GS
First -- check the ball joints for wear, if you haven't already. My Father-in-Law's old Gd. Marq collapsed at the front passenger side wheel, luckily while moving it in the driveway (and not on the highway). Same for excess buildup of carbon, which causes constant knocking and the knock sensor to burn-out. Also the intake manifold can be sensitive to fracture (all experiences from this one car).
Depending on how bold you want to go (someone else can back me up on this or not), but how about a 6-cylinder turbo (or even something like the 4.0L from the Explorer)? Usually finding the smallest displacement helps FE and adding forced induction when needed can help with adding some power when you need it, and low fuel consumption at cruise. Keeping your foot out it -- that may be another story
The first thing I thought of was the 2.3L Lima or OHC Turbo -- found in the Merkur XR4Ti, Mustang SVO (intercooled), and Thunderbird Turbo Coupe / Cougar XR7 (intercooled). The latter were heavy cars and seemed to work well with that setup (but they were at 3419 lb. vs. the Vic at 4129). The supercharged models might be a better fit, but not as easy to control for FE
Just brainstorming...best of luck...