I suppose my first question is, what tranny are you running? You say first ,second, third, and OD. I don't have a truck with a 283, but I run a stock 230+ hp 283 in a 60 vette, and at 70 mph, I run about 3000 with a 3:55 rear. Stock 4 speed with 1:1 final drive. I'm running 15" wheels, so there may be some difference in tire height and vehicle weight. I'm running a 500 cfm carb, stock motor, and it seems to be happy,runnig about 1800 - 3000. I drive on the level, and live up a steep canyon, and the 3:55 works pretty good. Some place to start anyway.