stem length with beaming?

[D Josiah Boothby]

I'm trying to make a few beams longer than they normally would be 
because I'm adding tremolo and the tremolo overlaps with the beam. Am I 
misusing \override?

\version "2.4.2"

Notes = \relative c''' {
   \time 6/8
   \override Stem #'beamed-lengths = #20
   a,!8:128\mf^\markup{ \large\bold{Un peu lent} (\small{\note #"8" 
#0.9 } = 66) }
   bes,!:128 as':128 g,!:128 cis:128 r
   \revert Stem #'beamed-lengths
}

\score {
   \transpose f c' \Notes
   
   \layout { raggedright = ##t }
}