Size of beam/rebar in U-block above opening?

[fulingbusen]

How do I reinforce the U-blocks in the U-beam above the opening?
My bricklayer has ceased to exist so I can't ask him and I've hired another experienced old bricklayer who doesn't know how it's been calculated.

The U-blocks are 350x190x300. Should I throw in the largest possible I-beam or 4 thick reinforcement bars?
Should I try to fill half with insulation (styrofoam) to break the thermal bridge?

 

[BSOD]

Thin insulation is available to lay in, I believe it is 3 or 5cm thick insulation.

There are also pre-made beams with a compressive strength of 5Mpa. https://www.se.weber/leca/lecar-balk/lecar-isobalk-350-3900

The U-beams I have seen have been with rebar, I don't think you need a cast-in I-beam, but it depends on the designer's requirements.

I usually let the beam extend 20-30cm on each side of the opening.