That was the clearance I had on my TR4 using the yellow fan and the original (TR4) hub extension, ran it for years like that, your milage might differ...
The fan itself is rigid and in my experience had very little longitudinal flex, the main worry would be longitudinal engine movement, specially if your engine mounts are suspect.
1/4 inch clearance is going to be a problem come "change the fan belt" time and that is why later (when I located a TR3 extension and crank bolt) I went with the thin belt conversion along with the TR3 hub extension.
My TR4 radiator (original) has two sets of mounting holes on each lower bracket, not sure if the TR4A radiators are the same.
I just measured the fan to rad clearance on my TR4 and it is exactly 1 inch using the rearmost set of lower mounting holes.
M.