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How tall of tires do you have on the truck and what's the rear end ratio.......37 inch tires and 3:08 ratio will make any engine a dog ........
Your 350 should be fine. You'd be better off putting some decent heads on it and ignition system. I agree with Rattler12 your problem is not likely solved by a 400 SBC.
sounds like its still apart. you could buy a block and change the cam and be putting out some power for 2k and know the block is good. or buy a 400hp/tq for 3k with a warrenty. me personally i would pass on parts that have been sitting if you can pay for new parts for about the same amount