Dude, I hate to say it, but if you're asking questions like this, you probably have a long way to go before you can competently put together an exo-cage. Start doing a LOT of research on roll cage design. You need to know exactly what you're doing when building a cage.
Check out the builds on this forum, do a lot of reading on this one and others. You need to be a skilled welder, fabricator, and designer to build a useful, safe roll cage.
Good Luck!