Actually it amazes me how many boats seem to have undersized blocks etc for things such as the main sheet.
We had a similar problem to your's (slightly bigger boat , 34') and what we have done is go to a triple setup (just like Alex has suggested) for the main sheet then we are using the old two block system main sheet setup to replace our old single block vang setup.
A relatively inexpensive fix, leaving us with equipment that Ms Wombat will find much much easier to use and also easier for me if I'm single handing. Smiles all round. (I hope anyway, cos we don't get her back from the yard until next week with her bright shiny new rigging, yeehah.)