Since it seems no one else has ventured any guesses for you, so...
From what I've seen there is just one fan shroud bracket available for both big and small blocks so that shouldn't be the problem...
Can't tell if you have a big or small block in your car as pictured, but you certainly seem to have a shroud that is of the big block design in use there...
There were different shrouds available for rads having different core thicknesses as I understand it and or A/C use, so maybe you have one for a thicker rad core or application than you are using it on - thus the gap - but even then it still looks set back by a large amount to me...
I would check out the various shrouds available and pictures of shrouds in use by posters elsewhere on this site to see if you can spot one that looks like a better fit not only for the core to shroud distance but what appears in your picture to be a shroud that covers the fan blade area too much (fan blades should sit half in / half out of the shroud end for best cooling efficiency).
Other than that I have no other suggestions and hope you get more expert advice...
Randy