Is there any later model auto transmissions such as the one out of an EL worth fitting to a ZL Fairlane with a 4.1 EFI? The reason I ask is it may improve acceleration and fuel economy.

The only later model gearboxes that will fit are the t5 manuals and the EA 3 speeds. The 4 speed autos (will improve economy but not acceleration) wont fit unless you modify the floorpan.