When adjusting trusses, our inspecters would send you off for an engineers report.
Cutting the top of the beam would likely not be allowed. The truss company could check your trusses and supply proper hangers and maybe engineer the beam. Or have them build a gerder truss instead of a beam, it could have tapered ends.
You are likely going to hand frame your roof, I would have them build the valley set to go over the original trusses, they are so easy.