The setup you're describing sounds awkward to use. Here's a link to a site that seems to have some useful info on these boats, incl a mainsheet modification that makes sense:
Stuart Marine Corp. - Mid-boom Sheeting - rhodes 19 cb mid boom sheeting.jpg
Perhaps this will be helpful.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)