Why a 1972 mustang fastback with a 351 Cleveland been having trouble getting the engine to turn over when the key turns something in the starting system makes a grinding noise what could be wrong?

The grinding noise you hear is probably coming from your starter. When the bushings wear out, the armature "drops" against the casing, making it difficult to spin without "grinding" on it. Starters can be rebuilt if you are mechanically inclined - you can buy a "starter overhaul" kit at almost any auto parts store. Before you buy a kit, remove the starter and inspect it - it might be too far gone from the grinding to overhaul. If that's the case, you will need to buy a new or rebuilt starter.