Ignition timing for stock cam/heads Magnum 5.9?

What's wrong with 4.10s/ nothing mismatched about that for the street.

That 300 is a sweetheart of a street engine, with a nearly flat powercurve from 4000 to ~5200, a long, long powerband, and good torque from 3000 on.
I see the cam is advertised at 250/264/110 and if in at 108 then the Ica becomes 53*. If that engine is a 9.0 Scr then the pressure at 500ft elevation become 155psi with a 144VP which is fantastic and should be fine with 87 gas. That, with 4.10s should be a ton of fun. Timing wouldn't be a big deal in a streeter. Even if the power timing was 4* short of optimum, you'll hardly know it cuz she is accelerating so hard in the first two gears and hitting 65 at about 5300, and about time to shift.
In the lower gears timing could be really slow before you notice it because the tires will be spinning ,seems like forever.
But where you will notice lazy timing is in cruising at steady state; the fuel economy will be poor.
3000rpm will be about 55mph with tall tires, and the timing there should be in the neighborhood of 55*. Or more. You can get 20 to 24 out of the Vcan, leaving 30/35 for the D to bring in.
With that set-up to continue to burn 87, I would target 10 to 14 idle-timing, and 34* at 3400 Power-timing. The rest is a crapshoot, you can play with it for weeks and unless you do before and after time trials gains will be hard to find on the street.

But if that is a 9.5Scr engine (I couldn't nail down the Scr from my sources), then you might be in trouble.