www.mustangtuning.com is where I got mine. Use coupon code 'stangnet' for 5% discount!
Free shipping to.
I'm not sure about how much those spacers really give you but they claim 1-2RWHP. I have one on my Accufab with my 70mm TB and made some pretty considerable gains. Every bit helps, and the MT spacer is cheap enough I figured it was worth getting when I bought my plenum.
Best price I've found on the T/B and or Pleunum has been on Ebay over any others. Might be worth a look. Just avoid anything sold by Mustangwarehousecom.....there have been a huge amounts of complains about his stuff and his service practices.