All this engineering seems alittle over my head but have you considered mountain bike components for your rear derailleur? I have Ultegra shifters and front derailleur but run XTR gear on the rear - gives me a 30 - 34 ratio. Very low. You will have to buy a rear derailleur as well as a cassette and new chain. But for $200, a good fix (Although uncertain how this all fits in to what you have done already...)