I think what is the actual problem is that the entire cab assembly is mounted to the upper frame, which protrudes from the front of the main chassis by about 600mm. The stirrups both support the upper frame to a vertical position above the main chassis and stabilise it fore and aft. Remember the firewall is mounted vertically above the front dumb irons forward of the foremost stirrup.
When you remove all but 4 stirrups (2 on each side) to this upper frame the weight of the now cantilevered cab wants to rotate around the prop which is only bolted to the front dumb iron. It seems that the only thing that currently holds the assembly up is the windscreen frame.
When you remove the windscreen frame the cab hinges down by deforming the stirrups. :(

