I've had alot of trouble trying to find the correct bolt for the belt tensioner on a '92 RS motor (that happens to be in my '69 C10), I found a bolt that would work but because the bolt sits into a counter sunk mount you couldn't tighten it...*#! I solved the problem the only thing is, the fix was so simple, I feel stupid about it now (because it's taken me over a week to sort this out)
. I used an old socket and ground the outside edge down far enough to fit inside the hole to tighten the bolt. The original bolt, I suppose, is an allen or torx head, all I know is I've had a heck of a time trying to find one.