The bolt together convertor('57) can be split a part and then the blades can be modified to change the stall speed.
There are article in the older Y-Block magazine how to adapt and modify Fordomatics.
If you want it to look sort-of stock with a cast iron case, you can use a later FMX, as there were high stall convertors and shift kit available for them at one time. FMX was pretty good trans when modified. I had a friend who's dad owned a transmission shop and had a modified FMX behind at small block in a '69 Mustang and it was one sweet shifting transmission.
A C4 works really good if you have a modified motor, but not as stock looking. Jerry Christanson has C4'sfront pumps machined to fit the original bellhousing if you want to retain the stock starter. I prefer the stock starter, as they have a unique sound.
Pat Fleischman