The best fitting skinny 17" tire is the 26x4.5 made by M&H, and its $140/piece.
I had looked into using 2 Cobra spares and getting 2 tires, but in the end it would have cost more than just getting 2 draglite's and running Kuhmo 165's.
Here's a couple options I am looking at:
15x3.5" Draglites - $260 (10lbs)
Weld lugnuts - $30
Kuhmo 165/80's - $78 (17lbs)
1.25" hubcentric spacers - $73 (3.5lbs)
Install/balance @ local discount tire - $20
So for $461 you have everything to run the front draglites and they weigh 30.5lbs total.
If a stock 17x8 weighs 20lbs, and a stock 245/45/17 weighs 25lbs you are saving ~15lbs/wheel and a whole lot of rolling resistance.
You can also buy the bias-ply tires from Mickey or Hoosier and cut another 5lbs off each wheel, but its gonna add $200 to the total cost.
I am also looking into buying 2 15x4's from Diamond racing instead for only $43/piece and putting the same spacers & Kuhmo tires on them. It would come out to ~$300, but the Diamonds weigh 20lbs, so the total weight jumps to 37lbs, so you are only saving ~10lbs/wheel over stock.
I dunno, just throwing out ideas