Sorry for the delay. If you don't have locking seatbacks, you'll not only have your force against the shoulder belt, but the seat back as well, and if the seatback pivots like early Mustangs do, toward the center, you could end up with a shoulder harness around your neck or, more likely, a broken collarbone.
When I said "spoiler" I was talking about front. Nothing even all that visible, just enough to direct airflow out toward the sides. An air dam below the core support would probably work too.
PS: I scored a pair of '86 Fiero seats with headrest speakers for my '66. The backs lock and recline, just perfect for a 4-point harness.