how do i change what part holds the tool?

When a tool is equipped a weld is created in the player's right arm called RightGrip. You only have to delete this and weld the part to the player's torso.

Make use of the Tool.Equipped and Tool.Unequipped events like so:

local TorsoGrip
Tool.Equipped:Connect(function()
    -- destroy the right grip
    -- create a new weld and initialise it using the above local variable
    -- weld to torso
    -- weld to tool's handle
    -- set c0 and c1
    -- parent to torso
end)
Too.Unequipped:Connect(function()
    TorsoGrip:Destroy()
end)