The simplest answer can be found on Google; 6-10 degrees Before Top Dead Center (BTDC).
If wanted, here is a little more information on timing our big blocks; Since the OP is working with a STOCK 383 we'll stay with that. Although these points will apply to many applications.
First off, the factory set the timing on these cars NOT for optimum performance, but for the safety of the engine. They had to be very conservative in the specs because they knew there would be crap gas thrown in, that there would be heavy loads such as 4 adults and their luggage (yes, even in a RR or Charger), that the cars would see every conceivable driving condition and atmosphere, and on and on.
So, the timing is very conservative by necessity. For those that KNOW how to drive these cars properly and keep good gas in them, there are real benefits to optimizing the timing.
Our big blocks whether stock or modified absolutely LOVE a fast curve and a lot of lead at idle. The bigger the cam, the more lead they will like, or more accurately; need. But we are talking stock in this.
Actually, discussing the initial setting is the tail wagging the dog. TOTAL timing is the key and then you work backwards from there to get the initial AND the amount of mechanical curve. Typically, stock big blocks will like something in the range of 32* to 36* of TOTAL (as long as it doesn't ping in high gear under load). Then you see what the engine likes for idle timing (and still not hit the starter when cranking hot) and then you know what the curve is. But that often requires shortening the slot on the advance. Although not difficult, most won't tackle it so they will be left with compromises. Much like the factory when making these decisions although for different reasons.
Direct Connection and Mr Gasket used to offer lighter advance springs for our distributors and for good reason. Re-curving our distributors has always been one of the first 'mods' we made to our big blocks. A faster curve rate and optimum total can really wake up a big block. But the advance rate HAS to be tailored to the particulars of the combo: rear gear ratio, stick or auto, 87 or 93 octane, etc..... Specific details that the factory could not always tune for in each individual combo, so they went with the safest settings.
With the 3.91s in my Cuda, I have as much initial as I can get by with and not hit the starter when hot starting. Well, maybe hit the starter a little, but still start. My current set up is 24* intial, (it would like a little more initial, but the starter won't take it) all in by 1,600 with a 34* total. I have had combos where I used to lock the timing all in and have a switch to leave the ignition off, start cranking, then turn the ignition on to start. But I prefer to recurve the distributor.
For stock, something like 15* intial, all in by 2,500 would be a good start IF the owner wanted to make the appropriate mods to the distributor (and monitor the results for issues). Otherwise, just stay with the factory settings.
And ALL street big blocks benefit from vacuum advance. It is not necessary, but is beneficial. And that is adjustable from the factory, too. You can put the right size allen through the nipple to adjust the spring pressure and thereby when it pulls the additional timing in. There used to be different cans available with different amounts of timing, but now you have to modify what you have and that is a bit of a pain. But adding about 10* of vacuum advance at cruise makes for a more efficient, cleaner running engine. And a tiny bit of fuel economy too as if we cared.
WOW! That is one long post and doesn't even cover it all.
Oh, and I almost forgot.......................use ported vac just like the factory. Manifold sourced vacuum is ONLY used as a band-aid to cover up for a poorly set up distributor.
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