Randy, roughly estimating...the radiator is, lets say 50% fin and tube, 50% air space. The opening is 100% air space. You could easily have twice the radiator behind the small opening and still get enough air through the radiator. This also fails to account for the turbulence through the radiator and the cleaner flow through the opening.
Roy, in know it's not apples to apples, but just for info: I'm still running a sad stock 230 radiator with a few tubes pinched shut on a 327. I ran a 400 for a while too. I had trouble cooling the 400 when I'd get the bottom third of the rad plugged with mud. Some days when wheeling in a lot of mud I'd retard the timing as much as I could and could get along okay.
FSJ brackets are at least a couple inches wider, were you closer I'd give you the whole front. I have a couple to spare.
This post is closed-captioned for the hearing impaired.