Ok, What have I done now... (904 problem?)

your probably just way short on fluid , Ive got a near empty 904 waiting to go in my car and when I checked the 68 service manual for capacity it said like 8 quarts IRC which is like 7.5 litres and thats with a normal pan I`m pretty sure the purpose of a bigger pan is to use more fluid , so I think your probably only about half full which would definitely cause soft /slow shifts when it shifts at all .