The January '72 issue of the 1970-'71 Parts Catalog lists:
for manual antenna (non-power)
2889 934 76.5" mast for '70-'71 a, b, c, e-bodies
2889 919 60" mast for '70-'71 a, b, c, e-bodies
3513 745* 31" mast for '71 c-bodies (* = part not listed in any previous parts book)

I do NOT believe the fixed mast is correct on any '70/'71 a, b, or e-body.
Doesn't the '70 Challenger "owner's manual" show recommended lengths of adjustment for best reception - AM vs. FM??

Which issue of Mopar Muscle??