My car has the same issue, the balancer has slipped. Mine is way worse than yours, so I need to get a new one. I just read the issues with the car and no power off the line. The first thing I will need to know is the cam specs, what is the duration at .050 on the intake and exhaust? That will be on your cam card that came with the cam.
What you are experiencing is extremely common with mildly modified engines on a lot of these old cars, especially on the ones that were at the end of the muscle car era which had tall highway/economy gears. Your car most probably has 2.79 or 2.75 gears, can't remember exactly what they came with. Unless you change the rear gears the car will always be a slug off the line. How bad of a slug, and what can be done to get it to be a bit quicker off the line will depend on how big that cam is. Cams change where in the power curve the engine makes power, the bigger the cam, the more power it makes in the upper RPM range and the less it makes in the lower RPM ranges. So, by changing the cam to a bigger cam, the car will loose low RPM power and gain high RPM power.
What can you do about it? First thing is to get the harmonic balancer changed to one where you can actually time the engine correctly. Then you need to recurve the distributor, you can see if there is someone local that has a distributor machine that can do it for you, or you can do it yourself with a spring/weight kit. What distributor do you have? From the photos that does not seem to be a stock points distributor. Recurving the distributor is quite simple and you can do it yourself.
Depending on the cam you have you may need to change the rear gears, and maybe even get a higher stall speed converter. If that car has the stock 2.75/2.79 gears, I would say that you need to change them to get any off the line performance from the car. If you have a cam that is anything above 215-220 degrees of duration at .050 you should probably get a higher stall speed torque converter too.
I will give you an example, when I was starting out to work on cars in my teenage years, my first engine build was on a 1972 Rallye Charger with a factory 440 Magnum with 3.23 gears. I redid the engine, and put in a cam that was 230 degrees of intake and exhaust duration at .050 with a stock toque converter. When I finished the engine, the car was a dog, and when I tell you it was a dog, this thing could not spin a tire to save its life. I changed to rear gears to 3.91's and put a 3,000 stall converter and it was a completely different car. The car would do burnouts on demand, you would hit the accelerator at 30MPH and the car would go sideways spinning the tires all the way.
There may be some carb tuning you can do, but right now the first thing that needs to be done is to get the balancer changed so that you can get the timing right and get a correct curve on the distributor. You basically want about 35 degrees total advance with all the advance coming in at about 2500 to 3000 RPM. I can explain to you how to do this, but you first need to get the harmonic balancer issue resolved so that you can even start on this tuning journey.
