I believe the '78 350 utilizes an HEI distributor. If so, then yes, you could have caused your distributor harm. The ignition module does not like to have a missing wire, so is more than likely fried. Luckily they are not hard to fix or diagnose. Since you don't have any spark, do the following to troubleshoot the HEI:
- Check the distributor for power at the connector on the side of the cap with the ignition on. If there's power, disconnect the electrical connector, then remove the cap (four "J" hooks at opposing points on the lower part of the cap).
- Check the rotor and the cap for excessive wear (terminals worn or burnt, carbon tracking, cracks, etc.).
- Check the coil tower for excessive wear (especially the coil button).
- Remove the top plastic cap on the distributor cap (four screws). Use an ohmmeter and check the coil positive terminal to the metal case of the coil. The reading should be infinity (see the diagrams from the link below).
- Check the coil tower and the negative terminal, the reading should be 900 ohms.
- Check the positive terminal to the negative terminal, the reading should be around 700 ohms.
- If any of these tests show drastically different readings, the coil is bad. If the coil is good, the cap and rotor are not cracked or worn significantly and there is no spark at any wire, replace the ignition module.
The ignition module is located under the rotor and has two bolts which hold it to the distributor. For some good instructions and illustrations on how to do the above, you can check here.