What is the part number for the spare tire piece to hold it down thats longer than the oem one?

well there are two, one for compact spares, one for full size spares, no?

Actually.....3. I bought the one with the longest bolt...Then i had to measure
 

and cut about an inch in order to get my S4 wheel to mount securely.

You can disassemble the short one that came with the donut spare .. then swap out the bolt for the
 

length that best fits your fullsize spare. I have one with ~2.25" of thread exposed .. part number on that assembly is 4A0 803 899