I'm not sure what you mean by cantilever? I can't picture the configuration. Are you sure cantilever is the word you meant to use. Cantilever would mean that one side is overhanging with no support at all.

I would think that the 8X12 would work but you'd only need one (bent) on either side of the 12 foot bay you are creating. An 8X12 tie beam every 3.5 feet seems like a lot of overkill. With two bents 12 feet apart, you can then add floor joists, about 11'10 inches long, between the bents every 24 inches or whatever it takes to support your load.

If my assumptions are wrong, please help us by adding a sketch so your design intent is more clear.