You apparently have two problems Damien.
First there is a switch, called a neutral safety switch, that keeps the car from starting except when in either park or neutral. It is adjustable as to it's position by way of loosening and tightening a single screw.
You also may consider rebuilding your shifting linkage to remove some slop caused by wear. You can use a Hurst shifter rebuild kit as the bushings used are the same size that Chevy used on their linkage. Just be sure to buy the steel bushings and not the white nylon plastic ones. If your rods are severely worn they can be welded up and then turned down again or ground with a grinder to be round enough to fit in the bushing.
Your sticking starter can be due to bad bushings in the starter motor, or from heat soak. With headers in particular, there is enough radiant heat given off by the tubes to bake the starter motor enough to cause it to swell so much that the armature rubs against the field windings of the stator. If your starter is bad, and you have headers, then consider buying a mini starter (Hitachi) instead. They move the starter further way from the tubes by virtue of their smaller size. Because it is a gear reduction starter motor it can crank over even a big block with ease, though but the small block version as there are differences in the number of teeth on the starter ring between the two engines.