What caused this?

I would definitely not stick with the stock 318 pump. My first car was a '74 Satellite Sebring Plus with a 318. I put 360 heads on it, put in a street Hemi grind purple shaft, and topped it off with a 600 Holley on an Edelbrock LD340 dual plane. It fell on it's face right after hitting 3rd gear. Fuel starvation. A high-volume pump and high flow fuel filter cured the problem. When it comes to fuel delivery you are better off knowing you have a little more than you need than you are hoping it will be enough.