Carb studs are what you need for the upper to lower if its like my eddy performer. You can get em by the 4 pack at autozone or advanced. Can't remember the sizes off the top of my head though. They should be able to look it up in the computer.
I wanna say 1.5", 5/16 studs with either a 18 or 20 count thread
If you have considered adding a phenolic spacer, they come with bolts and will help to seperate the upper/lower and keep the upper cooler as well as lengthening the runners into the lower.