Kind of slapped together
Join Date: Jan 2010
Location: Bossier City, LA
Factory should be on the emissions label under the hood; also, it's in the factory service manual.
For my 1991 5.0, it's 10* BTDC, with the timing plug pulled (to disable ECU advance).
You CAN run it 12* to 14*, but by 14* you're hard into "What? You only have 91 octane? I'll blow my motor!!!" level if anything else isn't quite right.