There aren’t very many places left that will do a Custom 200 Six Torque Convertor but Hughes Performance can make you one. They are located down in the Phoenix Az. area here is their Web site link.
https://www.hughesperformance.com/tc-stall/
When I talked to them around 3 months back about making me a 200 Six TC for the 65 project Mustang I am working on. They said their custom Street Performance Torque Convertor for a Ford C4 200 Six is made to order (it would have a 400 to 500 RPM Higher Stall) and it would be be
$507.76 this also including the shipping to my shop in Flagstaff, AZ. I didn't go any further on it since I didn't yet know about the size Input shaft for the C4 was going to used. The Strongest Stock C4 Input Shaft is the larger Diameter 1971 up C4 Input Shaft with 26 Splines on both ends. The 1964 to 1969 Input has a smaller Diameter with 24 Splines on both ends and then there is the odd one used in 1970 it has the 24 Spline on one end and a 26 Spline on the other. That one might let you be able to use the later Model Torque Convertor's (that are much easier to find) on a 1964 to 1969 C4 without needing to change out the Front Drum to a 1971 up one. They should also be able to make some higher Stall TC's too if you want to ask them.
Some other C4 items that you might look into for a Performance Built C4.
The 1964 to 1966 Duel Range C4 don't shift very well what I like to do to Fix that is to use the what Ford did and use the 1967 to 1969 Select Shift Valve Body and also install a TransGo Shift Improver Kit.
I found a used 1967 to 1969 Select Shift C4 Valve body Core Here.
https://www.ebay.com/itm/175492717672?hash=item28dc2e5c68:g:1qgAAOSwDPNjdXnD&amdata=enc:AQAHAAAAoDpI2SzyL5FQ/MxciKS1N/gixcKZAwZvVal75ioeQcQDV4RytrX6LMpOhN9n0IqN1sTaiCt/XQ7NpxH0IEtQpKg35uHktt3C2UqBj69Raty7FnVHCL3h4fdA21h784IHUnRbY3znUeZRFIFCcoXeZ7uLGaSOZbwTq76h19W6It6DHPhlQpAtIgFHsmkRf6dyWRmgf9fPUet/P3HUo/4VknI=|tkp:Bk9SR9TszbqQYQ
TransGo 47_1 Shift Improver Kit Info is here.
https://transgo.com/product-details/47-1/
The Letter A C4 Servo Cover and Piston gives you more Band Holding Pressure and also a Quicker Band Release. I found a used A C4 Servo here like the one I looked at.
Find many great new & used options and get the best deals for Ford C4 Auto Trans "A" Servo A Servo OEM Used Free Shipping with Spring & Bolts at the best online prices at eBay! Free shipping for many products!
www.ebay.com
I also use a Small Aux, Trans Cooler to help the Trans live its longest life.
Something like this, Hayden Rapid Cool Transmission Cooler part number 677.
$44.99
https://www.summitracing.com/parts/hda-677?rrec=true
Also 10 or 12 Quarts of Trans Fluid Type F, Good Luck