A picture of the mounting bolt pattern on the econoline bellhousing along with the casting code on the face.
I'm taking a WAG and say you have the original econoline bellhousing with the C1UU-6394 casting code and the original heavy duty 3-speed transmission (referencing your transmission picture). You therefor have the recessed flywheel with 8.5-inch clutch disc.
Unless your econoline 3-speed transmission has the toploader dual bolt pattern it will only bolt to the C1UU-6394 bellhousing.
If your transmission has the dual bolt pattern, then it will bolt to a C6xx or C7xx casting code bellhousing. You would then need the flat flywheel and 9-inch clutch disc.
I've attached pictures of a 3-speed transmission with the narrow bolt pattern and one with the dual bolt pattern.
I recommend you use the bellhousing, flywheel, and 3-speed transmission you currently have in the 64 Econoline along with the corresponding rebuilt/refurbished clutch disc, pressure plate, and TO bearing.
