How about this? Though they want between $600 and $720 or so:

http://www.usradiator.com/dodge-truck-1974-78-v8-383-440-radiator.html

Dodge Truck, 1974-78 V8 383/440 Radiator
NOTE: 1/2-1 Ton, Original Core 21" Tall x 26" Wide

FOR PART NUMBER AND PRICING, SEE BELOW.
Product Specs
Make / Model Dodge Truck
Year(s) 1974-78
Engine V8 383/440
Notes 1/2-1 Ton, Original Core 21" Tall x 26" Wide
Material Copper/Brass
Available Cores Standard Automotive
High Efficiency
Optima
Core Size 21 x 26
Available Rows 4
Cooler Size 12x1/8 NPT
Flow Type Downflow
Inlet Size 1-1/2"
Outlet Size 1-3/4"
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Dodge Truck, 1974-78 V8 383/440 Radiator