727 Shifts Down to 2nd at highway speeds

A '74 360 would have had a Thermoquad on it (or a 2bbl.) from the factory.
Post a pic of your linkage.
If you're not using the Holley linkage adapter for Mopars (#20-7), that could be part of your problem.
20-7.jpg
You wouldn't be getting the proper "throw" on your linkage for the part-throttle kickdown.
Also, make sure your trans. fluid level is correct- checked hot, on level ground, with trans in NEUTRAL (not Park, that will give a false reading).