I stumbled across this most enjoyable comedy, on YouTube.
https://www.youtube.com/watch?v=nnXP9Y9SFiw
Robert Stack (the Untouchables TV series, 1959) plays a role in this movie and also John Lund.
The movie is about a guy who impersonates a very odd heir to a family fortune, who passed away. but the heir's helper wants the allowance cheques to keep coming.
Thanks, Bill!