Cool Dave’s 1973 Dodge D100 Club Cab

timing chain slack. That's a "normal" symptom.
The test is to rock the crank back and forth against the valve springs, without disturbing the cam.
2/3 degrees at idle is not that bad.

The sparkport should have zero vacuum at idle. In fact, if you slowly bring the idle up in N/P, it might take more than 1500rpm before it tickles the port.

That distributor is NOT set up right for your application.
I'm NOT saying it won't work, just that the cam in it is too short.
If yur at 5/6 and only get 28/29, then the cam is 29 less 6 equals 23 degrees. To get to a good number of 34/36 with a 23 degree cam, will take; 35 less 23 equals an initial of 12 degrees of idle timing.
You need to try it. and crank your idle-speed back to something like 550 rpm(A guess will be fine) in gear/any gear. Then twiddle the mixture screws for best quality idle, then add 1/4 turn rich. Then gently drive away.
If the engine, responds smoothly, then yur in business.
Finally, shift from Neutral to Drive several times.
If the rpm change does not bang the trans, or stall the engine, and is in the window of 100 rpm +/- 50 rpm, then, again, yur in business. That distributor may be fine.
The proof will be that the engine does not detonate at any time, especially at WOT, and also not with throttle openings from modest to WOT below about 3400 rpm.
And the final test is that the engine shuts off cleanly with no tendency to run on, which should not be a thing, at 12 degrees.