If you can get the flapper off the shaft you can usually grind the high spots/welds on the shaft and slide it out. Then I tap both sides of the manifold with a size that fit the shaft bore. Then install button head bolts from the outside to seal it up.
That looks like a 67 torque convertor, so use the adapter in the register in the crank. What are the numbers on the pan rail of the 904?