Thanks for the video, Timo... seems you purchased a '57 BelAir sedan with solid sheet metal. Did you get a cowl plate with the car? It's most likely an original harbor blue/larkspur blue car, but the cowl tag will tell you for sure.
Has the body been off the frame? I'd suggest you spend some time *adjusting* both doors to the body before modifying the original sheet metal. Adjust the doors to the body, and then the front clip to the body and doors... There are adjustments and shim points to take care of *most* of the gap problems.
Good luck on your project and keep us all informed!