What size transmission do I have on my 1966?

I have a 1966 Dodge A-100 short van. it is equiped with the 225 slant six and has a 3 speed transmission with the shifter on the side colum. or AKA the "three on the tree" shifter. I am trying to figure out the size of this tran so I can find parts for it, I am missing the speedo gear and speedo plug and have also had no luck in finding them. Also what size is the transmission oem for the 170 slant six an will these part work as well? Is there any transmissions out there that are new that will work with a 225 slant six? I live in California Los Angeles County