I'm going to make my own valve spring compressor tomorrow for my LS1. I need a bolt to screw into the head so I measured the rocker arm stud and it came out to be 5/16 - 20. I've never heard of that size bolt... is that the correct size or did I measure wrong? Thanks.

 

The LS1 has 5/16th studs...sounds like you did your job right to me!