Pinging after 40-45 mins of driving?

[Mrturner1]

Hey guys, I'm havin some issues with pinging, but only after I drive around for a while. I let the truck warm up before moving it, and it runs great for a half hour to 45 minutes, and then it starts pinging and idling a little higher. I set the timing when it is completely warmed up and sitting at operating temp, and it runs and idles and drives great for a while, then after 40 mins or so it changes. Any thoughts on that?

It's a 1970 C10, 10-1 compression 355 with a comp 280H cam, RHS 170 heads, edelbrock performer rpm manifold and 650avs carb, long tubes with flows dumped. Also has a GM performance HEI ignition

Matt