Welcome to the wonderful world of FE/FC Holdens .
The engine # L419082 was produced in very late January 1958.
The #B engine is as you describe an FB engine.
If you find the Vehicle Serial number which should be on the upper passenger fender skirt above the battery.(might be on the drivers side)
This number along with the body ID plate details that you have already supplied should be enough for the Guru's (Ken) to date it accurately.
Good luck with the build and keep asking questions and posting progress pictures. (We all like looking at pictures)
Cheers Stewy