You would also need to redesign the whole transmission, not just one gear. Making a one-off cluster gear that meshes with 1st. would cost several thousand dollars. Since 1st. isn't synchronized like the rest of the gears, you'd still need to come to a full stop to engage it. Your checkbook will need CPR after that project. Most of us use a SM 420 as a 3 speed with a synchronized first gear, something a regular 3 speed can't do. "Granny gear" is not used often, other than getting a heavy load started moving on a steep uphill grade.
Jerry
