After looking closer at the cantilever soffit over the garage. Iv found it drops more than 1/2" over the door. So if I furr it I would have to shim and level it so it was level with the bottom of the outside wall siding.
Pain in the a##. And I like the hollow look and dont want the soffit to stand proud or even flush with the bottom of the siding. When its hollow you cant notice the 1/2" drop up 2" inside the soffit.
So the 5/8" #6 stainless pan head screws sound like the best option. I will only use these short screws on the bottom J channel which recieves the board and batten vinyl siding so they dont penetrate the back side of the T1-11 in the cantilever soffit area. And use longer screws or nails everywhere else. Then I will cap or wrap the bottom edge of the T1-11 with matching coil stock up to the existing soffit since no nails will be poking through. Then install new soffit over the existing to keep that hollow look.
My only issue now it the diameter of the screw. Vinyl Siding Institute states to use a #8 panhead or sheet metal screws, but the diameter of a #8 (.164") is more than the nail slot in my siding (.150"). So Im deciding weather to enlarge the slot (take forever) or settle with a #5 or #6 screw.