Riiiiight...

I'm calling shenanigans on this story.

1.  It was 47 years ago.  It's possible this has been embellished. 
2.  I doubt VERY seriously that anyone says "Let me see your face, you bastard." in situations like that.
3.  The President of the United States, the day after the assassination of the last President skulks through his former house and the Secret Service Agent who is there doesn't know it's him, or even acknowledge he was informed he MIGHT be in the house?!  I'm gonna go out on a limb and say that there would have been a whole team of agents and supervisors there telling him - "Johnson's coming.  As a matter of protocol - please don't cock your gun and aim it at him." 
4.  If this guy was close enough to Kennedy to know that he didn't dally with Marilyn Monroe, explain to me how the day after Kennedy's death he was put on night patrol of an empty Vice President's house.  I'll wait.  Right over here.  In the corner.  I won't sneak up on you, though. 
5.  Bullshit.  Just bullshit.  47 years later and the Kennedy mystery can still make anyone even remotely associated with it some money.  Good on this guy for finally cashing in.  His story is still bullshit though.