Usually Dwell all over the place means loose Distributor shaft bushings . You can see it plane as day on an Oscilloscope . See if you can wiggle the distributor shaft side to side at different angles , if you can then you know it’s shot

 

Vaux- did you say the tach drive gear was a little tight? Might want to check that,..when I bought my 72, the gear was shot. I replaced it, but over 30 yrs ago, so dont remember the details, but maybe you can temporarily remove it, to isolate a potential bind issue, and at the very least, to inspect it. I know you plugged your dist. Vacuum line, but whenever troubleshooting or as a pretune check on my 72, I Always disconnect and plug the larger vacuum line coming off the manifold, because the old vettes have so many vacuum driven accessories that could be leaking (headlights, wipers, pvc valve, etc).

 

my first though was worn dist shaft bushings, point rub blocks worn.
then a poster talked about condensor issue.
from your posts, sounds like condensor?

 

So when i had the distributor out again, i realised i had vertical play on my breaker plate. I had mistakenly thought it was just the plate moving with the play in the shaft, but when i held the top of the shaft and the bottom of the housing together to prevent the shaft from moving, the play was still there. I'd been checking for lateral play on the plate so hadn't tested the plate like this before.

I disassembled and removed the distributor shaft and sure enough, there was play between the top of the plate and the retaining ring. I put a small flat piece of metal over the top of the bushing to try and spread the load and gently tapped it down with a hammer until there was no vertical play on the plate but i could only just turn the retaining ring freely with a hint of friction. During the process i kept inserting the shaft to make sure there were no signs of the top busing going out of alignment with the bottom. No issues, shaft slides in seamlessly after closing the gap.

While i was as it i installed the new breaker plate i had to rule out any issues with the old plate, despite the old plate feeling fine. Now the breaker plate is rock solid in ALL directions. Re-shimmed the shaft end play once reassembled and happy with it now.

Now with it gapped to 0.018 i got it running. The timing gun is still struggling to give a stable reading, bouncing between 32% and 45%. If i try and close the points gap to increase the reading it doesn't seem to move. Might get the odd flicker into the 50% range but closing the gap further causes the car to cut out irrespective of adjusting the timing so looks like the dwell reading function is useless on the timing gun.

When i went back to my dwell meter which works when cranking the engine only, i was getting a much more stable reading now.

As my gauge only has 4 and 6 cylinder scales (weren't many V8's back in the day in the UK) i need to convert the reading.
360*/8 cam lobes = 45* periods, 30* dwell / 45* period x 100 = 66.667%
Working that backwards to suit the 4-cylinder scale on my gauge is 66.667/100 x 90* = 60* dwell angle (4-cyl). This aligns with the comment on the gauge scale gauge to halve for 8-cylinders. I wasn't sure whether i was meant to be aiming for 15 or 60 previously.

The gauge reading now varies between 55*-62*. Halving that gives 27.5* to 31* actual, which is in the right ball park, it spends more time around 58* so 29 degrees. I took the cap off and measured the points gap and i'm at about 0.016" which aligns with the dwell meter and the intial value of 0.016" if gapping by feeler gauge. Happy with that.

Given the reading is taken while cranking, not having the battery at fully charged could mean variance in the speed the engine is being turned over, so i've now got the battery on charge and hoping i get an even better reading with less bounce with a strong battery. Makes me wonder if my old battery was adding to my issues previously wile trying to take a reading. Think i'm going to have to find myself an analogue meter that works while running, any specific makes i should keep an eye out for on fleabay etc that folks can recommend?

Now just need to look at my timing and see why it wasn't even attempting to fire once i had the gap set as above. Need the wife to get home from work for that though so that's it for now. Feels like i'm slowly getting there