I had the tank out of the car and cleaned it when I put the sump in. I think this is surface rusty stuff and maybe some weld slag, etc from the sump install. I cleaned it out after I sumped it, but I bet gasoline loosened more stuff up in there.
The first pass was ugly, ended up by the wall, then I pulled back into the groove, and rolled into the throttle. 1/8th mph was 52 and the finish was 93. lol