How common for “J” and “U” Head on the same engine?

I've got a 70 Challenger 340 (A66) car that I have owned since 1979. It has the original numbers matching 340 in it that has never been apart, and it has a J head on one side and a U head on the other. Sucker ran like a gut shot cat too....lol. It's been parked for years, but it's early on the retirement "To do" list!