how do i reset timing marks on distributor after changing intake gaskets

The engine must be in 'closed loop' to set the distributor timing, but all you have to do is warm it up, ordinary operating temperature. Set up the vehicle to adjust base timing. Clip the power cables of a timing light to the battery terminals and clip the inductive pick-up around the number one plug wire. The number one cylinder in the 4.3 V-6 is the front wire on the driver-side of the engine. Unplug timing connector to prevent the computer from changing the timing while you are adjusting it. The timing connector is a single wire connector containing a tan wire with a black stripe and is located taped to the outside of the main wiring harness on the firewall or behind the front carpet on the passenger-side floorboard of the vehicle. Loosen the hold down clamp bolt (found at the base of the distributor) 1/2 turn using a distributor wrench. Start the engine. Adjust the base timing by aiming the timing light at the timing marks on the front of the engine above the crankshaft pulley and balancer. Rotate the distributor by hand while observing the timing marks until the mark on the balancer lines up with the timing mark specified on the tune-up specifications tag located under the hood. Tighten the distributor hold down bolt and turn the engine off. Reconnect the timing connector. Remove the timing light and test-drive the vehicle to verify it runs properly.

If you didn't do anything to it, don't mess with it,if you did,and you know what your doing,it can be done by ear