You may want to try calling D&D Performance in Wixom Mich. --> (248) 926-6220. I purchased my Tremec 3550 from them and they were very helpful. They sell T56's and the appropriate bells to fit mustangs - I think even the 351s, but not sure. You can also check out their website at
www.ddperformance.com
Good luck, E
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1991 5.0 LX Coupe - 38,000 miles
13.17 @ 106.14 mph w/ 2.138 60'