1976 will have to be purchased. '75 to '76 have a number of changes IMO it would be well worth the purchase.
Dying going from N or P into gear could be the idle tuning as several have mentioned.
The engine systems for '76 were set up for emissions control (carburation, ignition, emissions sub-systems, exhaust) for either US or Canada (or Caifornia).
The 1960s engine itself is not that different (solid lifters vs hydraulic) spark plug tubes and so forth.
But depending on what carb, distributor, emissions systems were retained from '76 will determine whether to use 1960s tune up info or 1976 tune up specs. The 1960s non-emissions tune has an idle air:fuel mixture of around 12:1 and fairly idle rpm.
In contrast the 1976 idle mixture was closer to 14:1, and used a higher idle rpm and a number of other settings all designed to work with the emissions systems. If it has EGR then that should not be in play at idle and cold running.
That all said, I suspect the 18* at 950 rpm may be part of the issue.
If the distributor begins to advance at 750 rpm, and the engine is idling at 950 rpm, then the 18* is not the initial.
18* then is the initial plus centrifical at 950 rpm.
Lets look at the '67 factory specs for a non-emissions slant six and plot the acceptable range for timing with
Initial timing set at 5* BTDC, 600 rpm.
View attachment 1716135307
Look at the rpm where centrifical advance may begin to add timing.
Advance curves for the emissions era often were quicker, but begin at higher rpm.
If you have the original distributor, you can look up the specs in the service manual or Sun card for that year. Otherwise, the only way to know the advance for sure will be to measure the timing vs. rpm from as slow as you can get the engine to run.
You can delay the start of the mechanical advance by
adding tension to the primary spring.
If the emissions packages have been tossed, and using the earlier carb, then the engine will run strong at 650 rpm with a relatively rich idle mix. But that rich idle mix will kill at catalytic converter if the car is so equiped.
As 66fs posted, tuning is iterative and the timing and fuel mix work hand in glove. You may need to fiddle with the idle speed adjustment (initial throttle position). As you make the engine stronger, less throttle should be needed.
Only things I would add is that fuel mix set in neutral will be leaner than needed in gear. Add a little fuel from best in neutral/park so it will have some power in gear. Procedure most use is turning the mix screws in from best running. When there is drop in rpm and vacuum, turn the mix screws back out at least 1/8, preferably 1/4 turn.
