Will be using the original heater boxes
The original heat boxes, if I am not mistaken work in conjunction with the outer steel shell surrounding the exhaust pipes/manifolds. This is the heat exchanger that gives you cabin heat. Your engine fan blows through this exhaust pipe heat exchanger to the J boxes. Not sure how your going to replicate this with a header. Jim is also correct, under the hood heat is your enemy. I doubt your original engine surround tin will go over the header. Even in the cold we get in the Midwest, if your working the engine hard in deep snow, temps are an issue.
Not trying to change your mind, just point out the obvious. An oil cooler would be an excellent idea, with the heat exchanger in the cabin with a fan on it.
In my climate, as much as I would like a header, it is not feasible in sub zero temps. I like to take off my coats in my machine when driving it.
Regards, Kirk