"sealed up tight" isn't the issue - any distance from the rad is fine for the fan, and the shroud doesn't have to close off the whole rad, but you need to have the opening that the fan sits in a fairly snug fit, with the blades centered in the hole [front to back] -
like this - you just want to force the fan to pull air straight from the front
the reason your truck heats up idling is the fan is too far back, the fan is pulling air from the sides and just thrashing it about, not pulling it thru the rad - the reason the temp is OK at speed is cuz air movement takes the place of fan forced flow
here's a pic of the style I prefer [yeah, the rad needs attention
], came from some 60s/70s GM car, allows better air flow at speed and does the job perfectly idleing in summer - note the fan is 6-8" back but right at the shroud opening and the shroud is within 1/2" of the rad, per the Generals specs
Bill
