If yours is the '82 F100 mentinoned in your signature, then you should be able to adjust the timing. All the manuals say that distributor should not be moved, but I know I had to get my 300 I6 timing changed in California to meet the emissions test. I've never done it on the six, but I would guess it means to have to pull the distributor and actually moving it one cog to change the timing. I don't think that you can loosen the 300 and turn the distributor like you can on the V8s. Just be careful and continuously check the timing whenever you make an adjustment. The timing should be checked with the vacuum hose removed from the advance pot and the vacuum hose should be plugged.

'82 F150 2WD w/351W (orig 300 I6)
Slik