You could fab welded-on flanges (aka "doublers") to bolt the removable section to.
Might be able to use the inner gussets at the fenders as a mounting point, depends on the car.
Or for that matter, fab a new crossbar that could also have an upper rad mount bracket, and bolt that to those gussets.

I once pulled and re-installed a SB in a Duster without even pulling the hood, but it was a really tight fit.
Had a Milodon pan on it, used a carb-flange plate & a tilting bracket, trans & headers in car, didn't scratch a thing.
While the OEM upper tie bar is weak fore-aft, it's purpose is side/side reinforcement.