Your car is way out of tune, or engine is really tired= worn out.
Mine stock 72 duster with stock 318, 904 trans and 2,7 gears was about 9 sec 0-100km/h (0-60mp/h).
That was with stock 2bbr carb.
Check ingnition, timing, and if you have compression gauge, do compression test.
If engine is ok shape best upgrade would be gears, I swapped 3.2 and it really waked car up. I wanted still get good highway cruising so thats why just 3.2.
After that its engine rebuild with more compression so you can run bigger cam, dont go too big, I have 9.5compr now and second smallest lunati voodoo cam, still good low end.
When i measured compression it was about 155psi cylinders dry.