Usually they will let you do the c-notch due to where you palce the axle. Depending upon tire size and highboy vs fendered, you might move wheel base up to an inch. And the notch should move with it. If you are nervous about doing it yourself, you might want to buy a completed one. I know The Hot Rod Garage (Tulsa OK - Jim & Jason Smith are a father & son shop who are really great people as well as long-time buliders. Jim has been building chassis for over 30 years) or Sachse Rod Shop (Dallas, TX - another father/son shop who have been treating rodders like best friends for several decades) would be able to help you. I know they can supply you with the rails if you decide to do it yourself.

Just make sure you nail it, the whole car depends on the foundation!