I think the one I actually purchased was something like 25 & 3/4'' wide x 19'' tall. I didn't cut open the support, just had to fab a few brackets to mount the radiator.
I think that it would be sufficient to cool down your big block as long as you have a proper fan shroud and fan setup. My small block does fine with it (using a flex fan, stock modified shroud, and HV aluminum pump).
- '68 Chevy Camaro (sort of done, but always subject to improvement)
- '63 Pontiac Tempest (work in progress)
- '72 Datsun 240-Z (back-burner for now)