Kind of curious why you want to box the frame before you know what it is you'll be dropping in there, and before you know your suspension needs. The frame is meant to twist and flex to absorb shock in it's stock state. If your not making any mods you could be doing something you will need to change later when your mods begin. Best to have an idea of the end result before hand. When you box the frame without adding crossmembers and apropriate suspension you risk damaging the frame. These can become work hardened over time and then restricting the movement(or shock absorbing quality) while retaining the suspension may give you problems. This is all worst case, and I'm sure lots have lived happily ever after doing just this thing, as well as it is your project to learn with. But on the flip side, patience and a well planned build are a favorable happy ending. Just playing devils advocate, and only my 2c.