You will be ok with the different size lines front and rear. It is recommended to have a 10 psi residual valve to the rear drum brakes and a 2 psi residual valve to the front disc brakes. Run a short line to each from the proportioning valve with appropriate size fittings which can be purchased at any parts store. From the residual valves you can use adapter fittings to connect to your existing brake lines. When I purchased my kit from TSM they did not have the flex lines at the time so I bought some from Jegs with adapter fittings to fit existing 1/4" lines. Worked perfect so I didn't have to run new lines. You just plug one of those 3/16" ports on the proportioning valve. Good luck. You'll love the disc brakes.
