[rpmerf]

Here is where I would start:
Timing. Set your all in and get your vacuum advance set. All in should be around 36*, vacuum is usually around 10-12*. You want to be sure your advance is coming in fast enough. If you are cruising 2500, but you aren't all in until 3000, you will not have enough advance. For vacuum advance, the real test is a cold engine, choke on, light throttle, through the rev range. When the choke is on, the engine is running rich, so you need less advance. My setup is somewhere around 16* base + 18* mechanical (34 total), all in at 2500 RPM, + 12* vacuum. You need optimal advance when cruising. When I am on the highway, I am pushing about 46* timing.

Next would be fuel. You have EFI so you can tune this exactly where you want it. At cruise, around 17:1 AFR should be best for MPG. Play around with leaning it out at low throttle / high vacuum.

Another thought is your gear ratios.
Calculating for 255/55R18 tires and a 0.70 gear ratio, here would be your RPM at 55 mph with different rear ratios
3.08 - 1,375 RPM
3.73 - 1,666 RPM
4.10 - 1,827 RPM

Do you have lockup on your transmission / torque converter?
I'm wondering if you are running it outside a range it is happy.
I wonder if there is a way you could hook your HEI into the fuel injection to get computer controlled spark. That would certainly allow you to optimize your advance at 55 mph.