Try
www.stangparts.com they have conversion kits. You will get the parts much cheaper if you buy them from junk yards/individuals. I paid $700 total for all of my conversion parts including a used (40k miles) T5. I'm not sure what the current price is at stang parts, but it's worth a check.
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Michael Black
Quantum Motorsports
Norman, Oklahoma
1988 Merc Cougar 5.0 HO, P&Ped heads, 2.25" custom mandrel bent dual pipes, T5 five speed tranny
15.43 @ 91.08mph (not shabby for a 3600 pound car)
60' 2.453 Street tires suck!!!