You can buy T56 conversion kits mustang specific, and as for using the T56 trans with a T5 Bell? No, its not going to happen. Look into D&D Performance.
Welcome to D&D Performance - The 5 & 6 Speed Experts!
Probably the only and true kit available.
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Ryan
1990 Mustang GT (In Process)
1993 LX Coupe (Daily Driver, but just choose not to)
2005 Excursion (Big Pig)