Before investing any more time and money have you tested the gauge to make sure it is suitable? There is a high likelihood it will not function in they way you are hoping and you will be wild as a hornet if after sorting through the electronics you discover it was never going to work from the start.
To answer a couple of your questions.
The gauge requires 18V for full-scale reading; the JAW unit has 5V output so you require an amplifier to boost signal voltage.
The amp must capable of providing an output voltage of at least 18V at sufficient current to drive the gauge. (The 741 has a current limit of 25mA which is likely to be borderline for an automotive gauge so you should choose something fatter.)
The maximum output voltage an amplifier can produce is (typically) a few volts less than the amp supply voltage. Or to put this the other way around; the supply voltage to the amp must be at least a few volts higher than the largest output voltage expected from the amp.
This means the supply to the amp must at least 22V, say 24V.
The vehicle supply is a nominal 12V, so you require a dc-dc converter to generate the 24V required by the amp from the vehicle supply.
There are numerous IC to do this, the following is an example.
http://www.st.com/stonline/books/pdf/docs/5257.pdf
The most difficult part of this exercise is mounting the individual components, as you are unlikely to have the facilities to cut a custom circuit board.
You would save yourself hours of time by purchasing these items as kits complete with circuit board and assembly instructions. Generally the kits are not much more expensive than purchasing the individual components and you get everything required in a single visit to the store, even so by the time you buy the electronic parts, a case to hold them, some gas to and from the store you will not have much change from the $80 you saved by not buying the special purpose gauge.