Stock 6* AMX 390 - Initial timing?

In the service manual it says "Ignition Timing @ 500 R.P.M. .... T.D.C. +-1".

Two issues here.
One is you should have it timed at 0 deg as the TSM says.
More timing advance will cause the problem you have and
pinging / detonation at heavy loads and high engine speeds.

Two is the heat stove in the intake manifold can be over heating.
causes can be a out of balance exhaust system, especially if it
is a dual exhaust or a closed heat riser valve on the passenger side
exhaust manifold.

Headers seem to aggravate the intake heat issue.

Back in the bad old days of cheap (or free) AMC cars I blocked the heat
cross over ports in the manifold with steel or aluminum inserts.
I machined them for a 1/4 to 3/8 thick block off to prevent burn through.

You live in Florida, you only need the intake heat for the choke operation.
Electric or manual choke will be needed. Or no choke at all.

You need to set it at 500 rpms if that is what the TSM says. Setting it at 800 rpms is meaningless, because it could be advancing a bit at those rpms due to weak springs and old age. As for initial timing, it should be ok with maybe about 5 degrees btdc, though I would start at the baseline of 0, as the TSM says. If my memory is serving me, the mechanical advance in the distributor is 32, which with an initial of 0 is a bit conservative anyway. Most guys shoot for a total advance(mechanical plus initial, no vac) of around 34-38.

Does the engine have an issue idling at the lower rpms? If it has a stock cam it should idle ok there, though you might have to set it a bit higher so that it does not stall when you drop it into gear. I believe that the stock idle rpms with an automatic should be around 650 or so?

Once you have it set to stock specs, then you can determine if there is a problem. The guys above are right, there are several reasons why you could have a hot cranking issue, but you need to know what is going on with your timing before you jump to conclusions. Then you need to check the things like chokes and exhaust heat risers. Once all of the obvious is working right, and IF you still have a problem, then you can start trouble shooting.

Chris

I know I'm a little late with this post, but here's what worked for me.  Delco points distributor on a 390 with an R4B and Edelbrock carb, mild (214/224) cam.