First, very carefully check the true timing; stock harmonic balancers that hold the timing marks are well known to rotate the outer part toward the retard position over time. I've seen some that were as much as 180 degrees out of true position. Most of us put a chisel mark across the face of the two pieces of a known-good balancer so we can tell at a glance if the 2 parts have begun to shift.
Second, try a solid state coil from the later Ford TFI ignition. It will work perfectly with earlier Duraspark series modules, and they're almost as cheap as failure prone cylindrical can-of-oil coils. Partially-failed stock type coils and their electrical connections cause many problems.
Third, some Holley street carbs are actually "emission carbs" even if not described as such and are designed to run lean all the time. I had this problem on one customer car and a Holley rep & dynomometer expert told me that such carbs will never tune perfectly on even a stock 351-C. I spent a week fiddling with a brand new Avenger carb and finally replaced it with a 'tuner' carb the same size. Bingo- ran fine instantly and gained 2 mpg on the road.
There are a dozen other possibilities- one is the common mistake of using the positive post of the coil as an easy 12V+ power source for add-ons like a Holley electric choke. This will cause undefined troubles and may actually act as a rev limiter in some cases. NEVER connect anything else to the coil!