Mike,
My suggestion, based on what I've had at the border, would be take the complete truck, frame uncut to the border. Talk to them before you bring the truck in. Beg an plead with the customs officer to look at it and assure you that the cut frame story is still valid, and what exactly the frame is. At that point ONLY would I cut the frame. I've had too many customs stories change depending on who you talk to. What you and I consider "frame" and what customs consider fame are NOT necessarily the same thing.
I'll bet they want the entire body tub cut and rendered useless or the story will change completely to some "as yet unknown" when you arrive.
I was told a similar story when I brought a '55 body (no running gear, no doors, nothing but the tub) up to Canada. "No frame, no problem". I worked with the guys in the states to separate the body and frame (took a days work, not to mention that I HAD to rent a trailer). I left the frame behind, and arrived at the border. There the customs guy comes out, sees the body less frame and says, "Thats a frame and must be imported like a regular vehicle." I say there is NO frame, look. He, being the customs officer says "Frame, period."
I'd wait till you are at the border before doing ANYTHING rash.