I have a Shimano Cardiff 301 and I am wondering if an Abu Garcia power handle will fit on the Cardiff and if will work.

Tried that before.  Plugging a power handle from Abu Revo Inshore in to a Shimano Calcutta 200 GTE, it did not work well.  The hole in the Abu Power handle is too big and  resulted  to a wiggle or play when fitted to Calcutta 200.  Same result will happen when plugged in to Cardiff.

Nope shimano uses a 7mm shaft and abu/diawa uses a 8mm shaft

I believe Okuma reels have the same size drive shaft as the Shimano reels. I bought a couple of Okuma carbon fiber handles,and had to make the hole bigger to fit on my Ambassadeurs.