The Muchmre film shows us Chaneys motorcycle position is the same as we see in frame Z-206
The Altgen's photo shows that Chaney was ALONGSIDE the Lincoln when JFK was first struck....
Notice that the shadow of the front upper tip of the Lincoln's right front fender is clearly visible on the surface of Elm street. ( the shadow of the staff of the American flag is to the rear of the shadow of the fender corner. ) Draw a line from that fender corner shadow up to and through the corner of the fender .
Now notice the shadow that is being cast on the surface of Elm street just beyond the left side of the Lincoln's front bumper..... and draw a line from that shadow that parallels the line you drew previously....
Does that line pass through the left side of the windshield of Chaney's bike? Yes?.... Then it is the Windshield that is casting that shadow and it should be obvious that Chaney is ALONGSIDE of the Lincoln.