I figured the cab mounts would be different and I'm actually wondering if the mega cab will fit on the regular cab long bed frame.
From what I've read, the mega cab with 6'3" bed is built on a quad cab long bed frame, both of which have a 160.5" wheelbase. That said, the (I assume) same quad cab is used on the quad cab short bed which shares the wheelbase of the regular cab long bed at 140.5". This would lead me to believe that the cab of the mega cab was possibly designed in a way that it fits within the same frame constraints as the qclb and if this is the same quad cab design as the one for the qcsb then the mega cab may fit a rclb frame.
Maybe I should ask this way. I'm wondering if the mega cab will fit on the lower section of frame before the frame rises up under the bed? The front of my 8' bed mounts on two tall brackets that come up off of the lower section of frame before the rise. Maybe someone could provide me with a measurement from the front body mounts to the back of the cab, or the back of the cab before a rise in the cab? That way I could measure against my rclb frame. Another measurement that might help would be from the front door seam, by the front quarter panel, to the back of the cab. I'm thinking the back of the cab will be pretty close to where the current wheel well is on the rclb.