Hey Paul,
...might be to bring your block on the end of the boom closer to the deck by injecting a dinghy-style 'strop' to shorten the purchase as much as possible... On my boat, by the time the main is eased all the way out, even such a modest reduction on a 5:1 purchase really adds up in reducing the length of the required sheet... I forget what the configuration on your boat is like, however, sounds as if you may not be able to do something similar, no?
Great shot of your friend's classic sloop, btw...
best regards,
Jon