Cooling issues 69 Dart GTS 340

Also, I have the MP viscous fan package that puts the fan's clutch approximately 1/4" - 3 /8" away from my new (last spring) radiator (Spectra).
That's a good package and a should work fine.
That's cutting the distance really close.

I had a similar setup on my car until a couple years ago. The radiator was a semi-custom '70+ style. I was able to get more clearance of the tank and the core by resoldering the mounts so the top tank was as far forward as possible. Got about 1/2" to the top tank.
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A hood seal and a fan shroud helped a lot when idling in hot weather (ie stuck in traffic).
Unfortunately the shroud I had on there was damaged by the previous set up - a Chrysler flex fan that came apart.
All I could do when caught in a bad traffic situaiton is keep the rpms up a little and try to keep rolling slowly rather coming to a dead stop. Obviously turn the heater on if needed. But that was only in the worst situations.
The car has shroud again now, along with a new radiator. Expect this one will last at least as long as the one in the photo (18 years) since it will get better care. :)