Lose all the smog crap thats choking up the engine. Don't forget that the '75 is a Big car. I'm sure you can dual the car. A buddy of mine has a 78 Coupe Deville and he got it dual. I know Edelbrock has a intake for this engine. And remember...its a BIG car.
There's tons of options for intakes, from customs to Edelbrocks to just porting the stock one. There's a wide range of cams made, and several companies that make bolt on headers.
The skys the limit, depending on how much you want to spend.