MN,are you shooting wood shafts?
I recently melted 20 grs of lead into a Grizzly broadhead ferrule an still could mount a 125 gr adapter in it.The adapter is about 1" long.I can melt 28 grs of lead into the hollow cavity of a 100 gr adapter.Maybe this will give you some idea.I guess it would depend on how deep your broadhead ferrule is and you can cut back your taper to whatever you are comfortable with.
I thought i remembered you were using wood shafts,if so,I bet you could bump up your 125's another 35 grs if you needed to.
I have also tamped lead birdshot in with a 1/8" punch,when I didn't want to heat a broadhead.It seems to work as well.
Tamp 35 grs of lead into a broadhead,stick your taper in and see how much has to come off the taper,then you will know if you are in business.Good luck with it.