What should the stock Merc thunderbolt ignition timing be set at on a 1989 330hp 7.4L. Looking for the most advance without detonation for more hp. Should this be set at Idle with base timing or is it better to rev up the engine and set the timing while it has full advance?

With the stock Merc ignition, you are limited as to what you can do. I would set the timing at full advance to 35*. Let the base end up where it wants to.

I doubt you will gain a thing by advancing it. Stock is 32* at full advance. Do a water test with a GPS and bump it to 34-35*.

I allready done that, bin there. NO increase at all. Stay at 32* and be on the safe side of detonation.

best wat to go, if you go much more then 34 total you will need to increase octane

At what RPM should the timing be at full advance?

If your module is Thunderbolt 24 * Designation V8-24
3750 RPM