Depending upon what you are trying to do with the car I would go log manifolds or the long branch(but I like stock appearing). Since you don't have the original drivetrain whatever you do won't really affect value so do what fits in your budget and what you like.

Looking at the stock exhaust manifolds pictured at Frank's Pontiac parts: https://www.frankspontiacparts.com/engineparts/pontiacengineparts.htm

It looks like the exit on at least one side of the '75 manifolds is in a different location which might cause an issue. So, fit of the '75s may be an issue in a First Gen and should be checked.