when ford had the cossie in mind, one of those things as 'fuel economy' where not on the checklist. It's antique technology compared to today.
A cossie is ment for power, and power comes at a price.
the yb doesn't like high lambdas, (melting pistons and such )
and your lsd won't do much favors either qua rolling resistance
maybe fitting smaller tires and higher pressure will do some enconmy favors...