Removed distributor to change head gas...

If the timing chain is still in the correct position its an easy fix. Put screwdriver down spark plug hole #1 and rotate the engine over by hand until you reach TDC e.g. piston fully up. Now your either on the start of the ignition/power stroke or at the end of the exhaust stroke. Im not sure of the firing order of your specific engine but basicly this means that the engine is either firing on 1 at TDC or its firing on its other piston. Just put the screw driver down the other cylinders and find the other piston thats at TDC. Put the distributor back in so that the rotor button lines up roughly with the post on the cap that lines up with one. Put the cap back on and try and start it. If it almost starts just trim up the cap a little (twist it by a few degree either way) If it sounds way out then pull the distributor back out and line it up with its mate piston.
Hope this helps

On some 4.3 engines, when the timing marks line up, the #4 cylinder is the TDC. So you may have to point your distributor to #4. And look at your cap, sometimes, where the #4 wire joins the cap, the wire "inside" the cap may run to the other side of the cap, so look inside to see exactly where your rotor should be pointing.