When the engine is not running the clutch fan normally will spin freely if you reach down and move it.
You really need a shroud, get an after market aluminum one or make one if the stock one will not fit. Shrouds are especially important at low/no speed because they develop something like a vortex or venturi drawing air across the radiator.
I have a 416 (stoked 340) and a 26" radiator and the distance is about 1/2 if I remember right ( can look later).
What T stat are you running, from your temps it sounds like you are running a 185.
70 Challenger RT 440-6 EFI, 73 Cuda 416-6 EFI
05 Hemi Durango, 01 Ram 4x4, 14 Ram 2500 4X4, 10 PCP Challenger 6 spd RT, 01 Viper GTS ACR, 52 B3B w/330 Desoto Hemi, 70 Hemi RR (under const)
9 x Challengers. AAR Cuda, 4 RR, 2 GTX, 4 Chargers, etc... (too many to list)