T56

[thunderbolt]

I believe the chevy t56 has a different input shaft than a ford. You might be able to swap one in that would work, don't have any idea, would call a trany shop. the t56 is large and typically requires extensive modification to the trans tunnels when put in old camaros, I suspect mustangs would be similar. Pick up a copy of the summit catalog, it will have some custom trany's there. You could use a tremec, they already make conversion kits for putting one in an early stang and they are reportedly quite stronger than the wc t5. If you do put the t56 in, tell us all about it because I am sure others would love to make that swap.