The first 2 pictures in post number 2 pretty much prove it is a Warlock also. If they are original doors for the truck.
As for the motor, 383s stopped in 71 I believe.
Now if the fender or radiator plate is still there get the code from it and run it for the options. The vin number will tell you what engine it came with.
E=318 2bbbl
MY BAD
Serial Number W15BE8J502096 Make Dodge Model W1 - W150 Body Style 5 - Crew Cab with Utiline box; Compact Extended Body Maxivan (Tradesman) Gross Vehicle Weight B - 6,001 lbs to 10,000 lbs Engine E - LA 318-1 V8 2V Year 1978 Plant J - Tecumseh Road Truck, Canada Wheelbase 115,131,133,149 Rating 1/2 Ton Build Number 2096