Formulas are gaining in popularity but will never have the demand of the 77 - 78 SE. If the plan was to really enjoy the car for a long time I would keep as a Formula and update the suspension...or make into a race car.
If you wanted to flip the car then I would do the SE Clone. If this a Pontiac 400 powered Formula you have a good starting point. Shaker air cleaners, shakers, shaker hoods, T/A fenders, etc., are still easy to find.