HOW and WHERE did you "test" it? Did you use a spark-tester with a spark-gap calibrated for HEI? Did you test right at the coil, or downstream somewhere--where a burnt coil wire, failing cap or rotor, or failing plug wires could steal voltage?
Was the coil hot--like just after the engine stalled?
Even better--Did you mist the top of the coil with water from a squirt-bottle to verify the case of the coil isn't cracked or carbon-tracked?
What is the resistance of the primary winding? What is the resistance of the secondary winding? Does either winding have high-resistance continuity to ground? An ohmmeter can verify a defective coil, but it can't tell you a coil is good.
IF (big IF) the spark truly is weak at the coil, you need to verify the power getting to the coil, the wire harness between coil and ignition module, and if those are good...replace the coil.