Buick Electra 1963 conv: Suddenly Not drives at D (drive)

[buick63conv]

Hi Friends
Buick Electra 1963: I have a 401 from 1964 in the car with Dynaflow.
Have gone with Electran every day after work but one day it could not run with the D (drive) but the L (lov)is fine and R (Back) is fine. The oil looks good.

Can it bee some troble in valve house?
Valve that is stuck and it is possible to remove the house by removing the oil sump?

One owner befor must has change the orginal engie to 401 from 1964 (K stamped on the engine)

But I have the orginal Dynaflow left (P N D L R) at my transmissions switches.

Some owner before must have replaced something to 401 (from 64) and Dyna Flow from 63 fit together?

Grateful for answers to any of my questions.

Could very little about Buick (Have had most Chrysler before, has a Windsor cab 59 left)

Joakim L from Sweden