Distributor advance springs

Agree with above that its easy enough to check for sticking weights.
But your missng possibly some key info.

A '74 distributor and a '77 distributor may have very different advance curves even when using the same cam.
I don't know that for sure, but its very possible because each year the emissions package was tweaked.
If your going factory, you need to pick the entire package.

65-70 mph in 3rd gear is just the beginning of what I call high rpm.
The question here is whether its rattling with the gas pedal floored or part throttle cruising/light acceleration.
At WOT or near WOT, then the vac advance is irrelevant anyway.
If the rattling is under cruising throttle/light acceleration then your experperiment has demonstrated too much timing is a combination of vac advance and centrifical advance and initial. I will guess that someone had 'hotrodded' the distributor by removing the secondary spring and/or too much initial for the setup.
Also, on '74 there was a timing delay. Bypass it, if you haven't done so yet. Just take the vac advance hose direct from the carb timed vacuum port to the distributor.
I bypassed the delay, but out of ignorance lol. I didn’t know anything about the vacuum amplifier except that I believed it to be some silly emissions component so I just hooked the hose directly from carb to distributor anyway lol. As far as someone modifying the distributor, maybe? The 77 distributor got damaged and I’m gonna try and fix it. The distributor is from an older slant I had laying around. Don’t know what year, but it’s pre-73 (it’s orange). But my dad drove the car to work for a short period of time about 10 years ago before parking it. He’s a mechanic so he may have done something. But I’ve been talking to him about this so I’d think he’d have said something