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

Make sure your filter & spacer are properly seated & sealed to the valve body. If there is any gaps in it the pump cannot pick up the fluid correctly & it will cause the fluid to cavitate(suck air) as it goes into the VB & through the pump causing loss in pressure. Low pressure = lazy shifts.
For older performance cars I use the type"F" fluid in all brands. All type 4 fluids have friction modifiers to help smooth out the shift feel of automatics. In a performance car we want a quicker firmer shift. The modifiers work against that. Type "F" had modifiers in it to produce firmer shifts. Synthetic ATF-IV in an older 904/727 is just a waste of $$ IMO. The type 4 fluid is probably not your problem here though. I'd look for mechanical trouble & check pressures if you can.