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.
It starts to barely knock right before every upshift, then once I’m in 3rd, it starts to knock on freeway speeds. I’m fortunate that I have alternative routes to work, but at least now I have time to mess with it. On the other hand, I know that the issues was WAY worse before I put a new timing set on it, which I did just a little over a week ago.