Some people build the X shaped centersection somewhat like the 30s & 40s Fords, but they don't hook the center together as good as Ford did. When building one, keep in mind what force is on the center when you try to twist the frame(like one rear wheel in a big ditch)
If you keep it simple and understand that twisting force, you can design something that will work fine. Some people might have the metal bent into a [ shape on a pressbrake, some might use channel iron, and some would use upper and lower X's made from square tubing. I am sure you must have seen different types on your search. I like the tubing deal because then there is room for exhaust pipes, etc.
Go for it