Cooling issue, but this time it's to cold...

I did not do a beret test on compression, the pistons are spec'd at 9.5. I found out by mistake that 87 octane will absolutely not run in it, but the CR is nothing crazy. I'm running Autolite 66 plugs right now and have about 100 miles on the engine so getting ready to change them, I have a set of AC Delco R44XLS plugs I was going to put in. I have no idea if that's what I need as they are for a stock engine and mines slightly modified, so help with plug selection would be great even if that's not the issue with cooling.