Speaking from experiance DO NOT USE THE 284 cam the truck will be a dog period no matter which way you install it.It plain kill cylinder pressure I'm using a very similar setup using a LD340 and 750 Holley dbl pumper. The Cam is a comp 268 high energy. I have tested seven cams and the Comp-268 has given me the best times in a 4350 racing weight car. With the 284 cam the car ran a 15.8 with 3.55s. The car ran 15.0 to 15.2 same gears Comp268.
The key to getting your truck going is in the timing and carb by tuning and gearing I got this car to go 14.30. The comp cam is out of steam at 5700 but will rev to over 6000. So key here is do not over gear!.Also run the total as high as you can and not detonate try 45 total. By the way this car is another example like you are doing using parts from my other car and making it run Have fun !