We put a T5 in the 66 but only have a 289. You can get a bullet proof Astro T5 that is good for 550 hp if you have deep pockets and are going to beat the car hard on the track.
http://www.pro-forceperformance.com/astro_a-5.htm (about $2200)
Useful info from my swap for you to view:
http://chris66dad.tripod.com/id38.html
http://chris66dad.tripod.com/id33.html
http://chris66dad.tripod.com/id32.html
http://chris66dad.tripod.com/id34.html
http://chris66dad.tripod.com/id44.html
I did a bargain change for about $1390.
Here was my parts list and price:
350 T5 transmission (used but good)
25 T5 bell housing (used but good)
25 T5 block plate
110 28 oz flywheel that mounts a 10 1/2 clutch
225 I went with a hydraulic clutch actuator
150 10 1/2 inch King Cobra clutch package
15 T5 clutch fork
35 ARP flywheel and clutch bolts
150 Transmission cross member and rubber mount
0 Possibly a new yoke (check fit first)
299 Check drive shaft for 1/2 to 3/4" free play (new aluminum)
5 New speedometer gear
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1389 total
The 300 aluminum drive shaft was not really required but was a nice addition.
I could have saved another 75 on the hydraulic slave bracket if I made my own. Modern Driveline has a nice one that is just a piece of angle iron that would be easy to make. check my link above on hydraulic clutch
I searched for a long time before I found a guy who was parting out his 89 Mustang. He wrecked the car but "said" the T5 and engine were in good shape. So far it has worked out. I did put a steel retainer and new rear seal in it ($40)