Barbi Benton -- Hugh Hefner's former girlfriend known for discovering the Playboy Mansion in the '70s -- thinks the famous home should stay that way.

Barbi tells us it just wouldn't be the same if the Mansion gets turned into a museum ... even though she admits fans would enjoy it. Besides, the neighbors in the residential area would hate it.

The past Playboy model also reveals how she found the iconic mansion and convinced Hef to check it out ... and how her mom played a role in him buying it. Barbi tells us Hugh's stipulation was that she had to move in with him ... which she did.

Hefner lived in the home for nearly 50 years. He died peacefully there Wednesday at the age of 91.