Really hard to start 351C

Bite the bullet and spend $35 to add an electric choke. Endless cranking will add extra gas which washes oil off the cylinder walls and dilutes the crankcase oil. You're wearing out the engine as well as the starter. Mail order speed shops sell the choke units if theres no store in your area. Be sure theres a heavy ground cable between the transaxle case and a bare spot on the frame. Do NOT wire the choke to the hot side of the coil unless you intend to automatically limit engine revs to about 4000. Depending on what you or others have done to the engine, you may need a gear drive starter like Tilton or McCLoud to spin the thing fast enough to start reliably, but you will ALWAYS need a choke.