I'm trying to extend ac ductwork to a storeroom. Here is where I am: 3 1/2 ton ac, air handler to 14x22 plenum to 3 branches (small) & to a 12 foot long insulated 10" x 10" main branch with 3 small registers to a 10 foot long insulated 6" x 6" branch with two small registers to a cement block wall through the wall w/ 4" insulated round duct (3' long) to a small register in a 8' x 8' x 7' high (well insulated) storeroom. Will this work? If not, why?

You need to do a manual D to know for sure. Just Google for (Manual D).