How do I smoke a rib eye roast?

I have a 4 1/2 pound rib eye roast that I want to cook in my smoker.

It is a real smoke pit with a side firebox. I will be using hardwood charcoal and mahogany for the smoke.

What would be a good target temp for the inside of the smoke box and for about how long?.

I would imagine a relatively High temp and short cook time for smoking but I want to go for a 140 degree medium rare center.