The box crossing the 8 foot opening is rather shallow (in height) and it seems unlikely there's really a beam in there. If there's no true beam there now that raises the probability that you can freely make the opening wider.
There are several things you could do to find out whether there's a beam in that box. One is to pull the drywall off one side and ...
Beams are sized based on 1) Deflection, (bending) and 2) Shear. Notches can affect a beam differently in those two situations differently.
1) A beam deflects most at the center of the span. This puts the fibers along the top and bottom of the beam in the most stress.
Basically, those fibers along the extreme edges are stressed the most at the center of ...
The lap joint is to position the top surface of the beams to the same level, and to provide a common surface for fasteners.
The ends of most beams can be reduced some without diminishing the maximum load capacity - usually found at the middle of a beam.
Either the plate with bolt holes as suggested.
Or you could bolt some blocks to the floor on each side of the beam to stop it moving - especially if it does not have a plate already.
But some method of stopping it moving too easily is sensible - "just in case"...