masthead sheaves
 

I have a round freestanding aluminum mast that tapers to 3" at the tip and has a premaid masthead sheave box riveted in a slot cut in mast. My question or problem is that I want to install a larger sheave to use 3\8 line the original sheave is only big enough for 1\4 line
1|4" halyard to raise a 317 sq ft sail seems a bit light to me.
My question is can I use one of these Ronstan exit sheave boxs that are used for halyard exit at base at the masthead ? the one I want to use is rated at 1100mwl

Thanks for your help
Dave Huard