Better acceleration?

3.7L isn't small for current engines. One change is to modernize it with MPFI. One company began fabbing a custom aluminum intake for that. You would also need an engine controller. The Holley Commander 950 I play with allows that, but is older (UI software requires Win XP). Holley (and others) have newer controllers. Some use Megasquirt. Cheaper might be a TBI on a 4 bbl intake, but doesn't fix the fuel maldistribution problem of a straight-six. For boost, I might wait until electric-motor superchargers get better and cheaper. Some of the >$500 ones today actually work, but cheaper Chinese ones don't. MPFI works fine with boost, as long as your MAP sensor can read >1 atm.

Your note of 3000 rpm at 75 mph reminds me how bad my 1985 M-B 300D is geared. It runs 2800 rpm at 60 mph. My 1984 300D is even worse, running 3000 rpm at 60 mph. When getting on the freeway, you count the shifts and keep waiting for that last shift as the engine starts whining, then realize that is the tallest gear. Every owner complains about it. You think that exquisite German engineering could have used 4 gears more wisely. I think 1st gear is so low you could tow a tractor if the wheels don't slip. Anyway, I wouldn't change the diff gears unless you want to make your car a drag-race toy.