Well it was an idea anyway. I've searched and the only person missing person I found that might use a nickname "Bell" doesn't look like a match to me. But I'm posting her for consideration in case someone sees something I don't:
Belinda VanLith, 17 years old, missing from Minnesota since June 15, 1974.
She has blue eyes like the UID but her hair was blonde. Hair color is easy to change, however, or subject to interpretation (Some might call dark blonde hair brown). Her height was listed as 5'5" while the UID was 5'8" but it's possible for a 17 year old to have a late growth spurt and grow 3 inches in 2 years (or the missing report could be off on her height). Belinda's weight was listed as 110 and the UID was 150. Also possible for a runaway to gain weight or weight to be misjudged.
However Belinda's case says foul play was suspected which may rule her out completely.
Could she have used the nickname "Bel"? Or maybe she started to say her real name once and stopped herself only saying the "Bel" part of Belinda? Maybe others heard "Bill" and she went with that?
Little Eagle Lake, MN is about 730 miles away from Highland park, MI which seems really far away. But it's still in the "Great Lakes Region" and that distance could easily be covered in 2 years of hitchhiking.
All that said, I'm just not sure if there is a close enough resemblance:
She might have a similar nose but it's hard for me to tell. I'd like to find more photos of Belinda for comparison. I also think the fact that her case says they suspected foul play means Belinda is unlikely to be a run away. If anyone sees her as a good possibility, please submit her. Thanks.
The National Missing and Unidentified Persons System (NamUs)
ETA: Belinda's thread:
MN - MN - Belinda VanLith, 17, Little Eagle Lake, 15 June 1974