Lets see those pictures. If It was mine and I wanted to keep the 3 speed I'd hunt for one of the Borg Warner T85 Overdrives. They are as close to bulletproof as you can come and the overdrive unit is the R11 version which is also strong. Ford used them at least into the 70s. It should be a bolt in. There are sites that can help with wiring. There is a way to wire in a switch that can give you OD in all three gears, handy when towing.
I'm not a Ford guy but I have deep respect for those engines. You should hear from some of those folks soon.
