According to "The Best of the Harveyville Fun Times" (pg. 162) The two met in Richie Rich & Casper #9 (The Happiness Balloon). I don't kinow what other appearances, if any, he made. He tended to stick to his own milieu. As I said, Harvey did not get deeply meshed into the idea of a shared universe, and Atlas-Seaboard not at all.
I actually dismiss all the Harvey efforts as fictions written & drawn by Harvey, with Casper as a staff member, a sort of "Casper Code Authority". When he agitated for more control over the stories, he got dismissed. Later, he was about to take Steven Spielberg to court over that movie he made, and at that point,my fan-fiction starts.
One of the pics in Harveyville Fun Times shows Spooky recoiling in shock & Horror at a boxer using a severed human head as a punching bag ! This hideous image was simply to indicate that Spooky's artist also did horror comics in a more realistic way (flesh rotting from radium rays...)