Amanda Bynes -- Asked for 'Miley Cyrus' Grill ... Says Ben Baller

Amanda Bynes secretly wants to be just like Miley Cyrus ... at least in the grill department ... so says the man hired to construct her gangster bling, Ben Baller.

Ben is the artist making Amanda's mouth piece -- which he says cost her $10,000 -- and recently Instagrammed a mold of the actress' chompers. So when our pap saw him out in NYC ... he HAD to ask ... what was it like working with crazy the future rapper?

Ben says it was a trip -- watch the video -- he goes into detail about her nutty behavior.

But more shocking ... Ben also revealed Amanda's number one request for her grill, explaining how she wanted it to be just like Miley Cyrus'. Interesting considering Amanda previously blasted Cyrus on Twitter as "Ugly."

Talk about a 180.