MOSFET load switch design help
Extract from ATP304 data sheet: -



Power dissipation is typically 3.25 watts but assume 4 watts or more as an acceptable extreme.

However, I would drop that device - the data sheet doesn't give the thermal resistance when mounted on a PCB hence, it's impossible to design this device into an application. It's going to be dissipating over 3 watts so, without the thermal data it's guesswork. Choose a different part is my advice.

When you have found a device that suits, get the thermal resistance and back-calculate what the junction temperature will reach with 4 watts of dissipation.


  Is it a good idea to use ULN2803A to drive the P channel ATP304 gate?


Well, ignoring the issues about the ATP304 that make it unsuitable yes, using a ULN2803A is fine for driving the gate (10 kohm pull-up required of course). But there's a wider issue in automotive environments to do with excessive voltages due to load dumps and cranking the car producing significant voltages that can fly-along the 12 volt system and wreak havoc with inadequately protected electronics: -



Picture from Self-Protecting MOSFETs Deliver Improved Reliability in the Harsh Environment of Automotive Applications.

There is also the issue that most (if not all) car manufacturers use what is known as high-side switches for these types of applications and frown upon the poor unreliability of an unprotected MOSFET: -



Picture from here in an article entitled Automotive Smart High-Side Switch by Infineon.


  I would like some assistance designing a MOSFET load switch for use in
  an 12 volt automotive circuit to run a fuel pump controlled by a
  raspberry pi.


So, if something goes wrong and the MOSFET goes short circuit, it will continue to run the fuel pump until what happens? Most relays (yes, relays) that ran fuel pumps require a pulsing signal to keep them activated thus preventing a software issue keeping them switched on all the time. This was common place in the 1980s although I can't speak for all vehicle types. I can speak for Audi though because I had one and the fuel pump relay failed and it would only activate for ten seconds before cutting out the engine. I swapped the relay with a non-pulsing type (probably for the headlamps) and drove home and fixed the broken relay with a bit of solder and swapped back.

It's up to you what you use but, be warned about power dissipation and using unprotected MOSFETs that can go short circuit when failed.