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.
".. 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)
1984 Fast/Nicholson 345 "FastForward"