First and foremost, have you checked the fuel pressure? What about the filter? Fuel injected motors don't stumble and gag when the pressure is low, they just run out of power from being lean.
Now then. Smallblock Mopars are hard on chains due to poor oiling,hence Mopar went to a double roller chain with a tensioner. It's also possible the outer ring on the balancer has slipped, and your timing marks are off.
Replace the chain, and here's how to check the balancer while you have the water pump and all the rest of the stuff outta the way. Knock the guts out of an old spark plug and JB Quick a bolt about 3" long in the shell. Screw the piston stop you've just made in the number one cyl. Gently roll the motor by hand until the piston hits the stop, mark the balancer. Roll the motor the other way until it hits the stop, mark the balancer. Remove the stop and measure between the marks. Top dead center [0 degrees on the balancer] should be half way between the marks. If it's right you're golden, if it's off replace the balancer.