When I converted from a R sized main to a F16 size main I had a similar problem. However, I found that the boom was just long enough, I think I moved the through boom pulley to the very end of the boom or purchased I different type of pulley. It did work. That was on Stealth 516 and it had a rectangular alloy boom approx dim's 65mm x 45mm.
During the 2008 Global Challenge I bent Stealth's 555 alloy boom beyond repair, instead of making an insurance claim I made a new boom from the bottom section of a Windsurfing mast. It works well 99% of the time but there are the very odd occasions when a big gust hits I feel as if the boom bends slightly.
Way back in 2005 I remember racing behind John Alani who was using a part of a windsurfers mast as a boom and that bent like a Banana. So be careful, if you are willing to go down this route but can't find a suitable mast let me know I should be able to help but you'd still need to source a new goose neck fitting as the old Stealth's had a cast alloy affair.