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.
