85eurocarrera is correct. If I remember correctly the seat rails were about 20mm higher starting in 1985. I had a 1985 coupe a couple years ago and had headroom issues too. My solution was to sell the coupe and buy a 1987 targa. I've never looked back!
Short of that, you may be able to change seats and gain some headroom, but the issue is that if you mount them with sliders etcetera you may not gain as much headroom as you would otherwise hope for. Your best bet if you love the car is to have someone professionally lower the seat rails. Just beware that the fuel lines run through that center tunnel, so have a shop familiar with Porsche 911's do the work if possible.
BTW, I see you're from Lansing, so we're only about an hour away from each other. I hope to meet you at a PCA Iroc event this season!