this is very hard to answer.
if you have a stock motor that has not been touched then you can use the manual as a guide line.
guys run Cleveland from 4 BTDC all the way to 25 BTDC depending on application.
if timing is too low the engine will overheat at idle. if timing too high it will ping/detonate or doe something even worse depending on load when the engine grenades itself.
as a basic guideline you can start as the manual suggests 6-10 initial is the average. what you want to do is check the total timing with the the vacuum advance disconnected. you want to make sure the mechanical timing advance is not higher then 38 degrees 34-36 is safer. when you bump up the initial timing you are also bumping up the total timing by the same amount.
so there is a balance if you want to run more initial timing because you make more power off the line with advanced timing then you need to limit the full range of the mechanical advance.
the mechanical advance has a stopper plate that cane be changed or modified to reduce or increase total timing. they are marked as 10L, 12L 14L, 16L etc. the number tells you how much mechanical advance each plate gives you. 10L is 10*2 or 20 degrees 12L is 24 degrees , etc... so if you take 6BTDC+20 you get 26 degrees of total mechanical timing. well below the recommended 32-34 range, or 38 or more on some cars.
so you have a choice you can either bumper inital timing up to a point or swap the mechanical stopper plates with a wider one. if you have a 14L plate, that is 28 degrees and your inital would be 6-8 BTDC giving you 36 degrees of total timing.
so you can see it is a hard question to answer based on the information. a modified motor could be anything but safe is 6-12 degrees of initial timing usually, and you check the total mechanical advance after adjustment.
you can limit timing by using a piece of emissions hose on the mechanical arm that hits the stopper. by using different hose thickness you can tune total timing if needed. what you would do is set the timing, go for a drive without the vacuum advance and see if the engine pings, if not you can try to add a little more timing but double check the total timing by reving the motor and using the timing light. many guys bump it to 16 degrees as they claim Cleveland love advanced initial timing, i don't know how true that really is. though more timing means more burnouts from the rear tires.
