Yes! There is one VERY big exception.
It is 2.5L Automatic with the AW-4 automatic that contains 4.56 natively.
1984-86 will have 3.73 ratio with the automatic (IIRC)
2.5L Auto 1987+ will have 4.56. They ARE very rare. I mean you'd have better luck searching for Hens Teeth.
That being said...
The RENIX ecm needs to see 300rpm + at the Crank Position Sensor, located behind the head (top-right of Bellhousing if standing at the front of XJ) before it will fire.
Upgrading the batt + cabling and such will make it start quicker, yes. Less internal resistance = more voltage arriving at the...
Im not sure on the newer DIS style engines, but 4.0's were keyed on the distributor. It IS still possible to stab it in incorrectly but you'll have to verify what I just said, Im not too sure, but it is an iddea.
Its carbed. Best economy will be what an XJ gets fully loaded and in the city.
Dig an MJ. Its only a 1/4 ton pickup but it'll work for your cause.
If you dig a J-10, the usual drivetrain is 258/4spd/NP208 but 360/TF727/208 rigs are usual too. They're carbed, big tanks but they like gas and...