new to me 68 Barracuda Conv, sluggish 318

The throtte cable bracket is from a 2 barrel car. you would be better using a 340 bracket that is available. The kickdown linkage is not correct and not adjusted properly. A cable system is the cheapest and easiest at this point. Get new wires, cap, rotor, and spark plugs. Set initial timing a 10 degrees BTDC to start. Do you have J heads? You should be able to see the J between the spark plugs. If not pull a valve cover and get the numbers off the head. Make sure your carb is open all the way when a friend floors the accelerator. That should get you in the ball park till you go the next step.