I just tuned some 5575 Trads to a 53# Mojostik.My draw is 27" and arrows are cut to 28".I have 290 grs up front,head+insert.I'm just making a very wild guess and thinking you may need about 100 grs less on yours,due to 2 more inches of shaft and probably 9 more #'s of draw weight.
I couldn't use the 100 gr inserts because I needed to use a steel insert with the broadhead I chose.
Your results may be entirely different but if it were me,I would want to be able try heads on the lighter end first- 85,100,125.
Awhile back I was messing with some 30" aluminum arrows with 100 gr brass inserts.I tried everything from 125 to 250 grs and was having fits.I couldn't believe the shafts were weak and thought I might be getting some opposite reaction and was reading it wrong.
I floundered for a long time till I discovered some 100 gr heads I forgot I had.With those,the arrows shot great.Now I had somr direction and new I could cut some shaft length and get the point weight up.
If I hadn't had those 100 gr heads on hand,I'd probably still be beating my head on the wall.
If you do find that a lighter head is needed and if you can't get rid of any shaft length,you might have to consider using the aluminum insert to give you more flexibility in broadhead weights.
Again,I'd be afraid to predict what would work for you but a good assortment of point weights can help you narrow it down quickly.