Question: Shudder at light throttle around 40 mph

I wouldn't throw parts yet. A pressure test beats staring at a dry driveway if the 1968 Ford Mustang is losing coolant with no puddle. Don't erase codes until someone who can help has seen them. I'd want a dollar figure or a measured spec, not 'should be fine.' Worth doing even if the car feels fine right now. This is me being the wet blanket on purpose.

Start with the test that can prove you wrong. Move a coil or plug on the 1968 Ford Mustang if it's cylinder-specific and see whether the code follows. That's the branch I'd chase this week. Happy to compare notes if your setup is different. Worth doing even if the car feels fine right now. Parking it somewhere I can hear it better next time.

Ask what changed right before it started. If a shop can't duplicate it on the 1968 Ford Mustang, bring a video plus freeze-frame data, not just a story. If the cheap test changes it, stop and reassess. I'd want that in writing, not as a promise. If the story changes I'll post the new data.