What little I know, your 2 hole valve cover indicates a 216 motor or a pre 54 235 or a 235 with studs added to receive a two hole cover. 1954 235 and up have 4 hole perimeter mounting not center 2 hole. You said is was 1955? something's not right. Their may be variations on big trucks and car motors regarding valve covers.
I see smoke, but its' not filling up the room/area? There are a lot of old trucks that smoke that much. I could not tell, because the camera did not stay on water drops, if it is seeping out or dripping from expansion plug or what. It does seem to be bubbling out but there is water forward and rear of the major area?? As said, the smoking has nothing to do with the water leak. Pour some stop leak (like BARS-Leak) in and drive. If you leave the cap off the water will be gone quick. Leave it on.
Forget welding, that takes heating the block first even if silver solder or brazing is done. It should be done with block out and stripped. Oil will come out of the casting and contaminate the repair, etc, etc. There is a HIGH risk of cracking it big time.
It would be nice to really clean the area and wire wheel it to observe type of crack or see if it's pin holes.
