727 Shifts Down to 2nd at highway speeds

Looks to me like 2bbl linkage on your 4 bbl setup.

You can see your slider at the throttle Is angling up from the corner turn buckle.

Needs the 4 bbl manifold throttle cable and kickdown bracket. Not the 2bbl bracket.

So your geometry is off and looks like it is binding up tight at the turn buckle causing it to stick in the higher pressure setting, instead of backing off slightly when you are cruising with less pressure applied at the kickdown.

Too much pressure will make it want to drop down a gear.