Does anyone know for sure if the power handle from a tranx will work on the new curado 300. I assume it will just trying to make sure. 

I think every Shimano except old Bantams and new Conquest are 7x4mm handle slot.  

https://japantackle.com/tuning-parts/handles-casting-reels/shimano.html

Handles on this page accept 7mm main shafts, which is employed by all Shimano bait casters 50-400 size, except following reels.
1. Shimano old Bantam reels. Shimano 00/15Conquest 300-400, TE/TE-GT 300-400. They have 8mm shafts. Please pick handles for Daiwa/ABU which accept 8mm shaft, and M8 hooded nut.
2. Calcutta D series 2012- Out side drag model : Please choose specified handles.
For 00Conquest 200 and Calcutta 200TE, please add a 0.5mm washer between handle and star drag. If you are not sure if the tuning parts fit to your tackle, please Email Jun Please refer to here as well. 

 

Check the part numbers of both shafts, im betting they are the same. If so then yuppers!