Back in the 90s I know I managed to kill cars using only the 45... maybe they just had really low armor, because I just tested it now and sure enough, I could never kill anybody with the pistol until they were in the red. But if I got their bar to red, it didn't seem to matter where I shot them - front, back, side... within 2 or 3 shots, I always heard the telltale HOOOOOOOOOOOONK of a body slumped on the wheel.

How I completed the mission:
1) Don't enter cutscene. Shoot the cops with Firerite once they're in sight. My 2 primary front weaps are Firerite and 20mm cannons. Rear weap is Mortar.
2) Lead the police cars to the mountain range on the left of the highway entrance. They will usually just end up getting stuck on the mountain walls. Then just blow them up easily.
3) I ran out of ammo when I finally got to the last 2 cars, so I just Mortared them to death.
4) Helicopter crashed into mountain I think, so it wasn't a problem for me.

Holy shit I did not know this and it's really fucking useful.
A thousand blessings upon thee.

Always had aim-neins for that mission to take out the chopper, but in order to finish off the cop cars, here's another tip (if you're desparate like I was)
head down bauxite road, and at the cross roads you'll see a shed with an open door. Drive into it and wait. Use F5 to watch the cop cars and E to flick between them. They go completely nuts when you drive in there, they can't shoot in and they keep driving into walls. Eventually a few of them will flip and explode, (although 2 re-enforcements do arrive) and you'll be left with a handful of damaged cars.

This is probably the hardest mission in the game. At least the hardest of the first 10 missions. I did pretty much the same as what is written above. But it's still really easy to get iced by the cops.
The idea is NOT to try to shoot down every single car. That's just suicide. Go to the barricade, shoot down one of the cop cars, but only one, and do a u-turn and then drive to the dirt road area in the northern part of the area. There are a lot of dirt road, hills and ledges they can crash into. The computer AI is pretty dumb which means that they drive like shit on dirt roads. In that way they all kill themselves by crashing and you don't have to do a thing :)
The chopper? If it doesn't just crash by itself, you should go to the sourthern area, where you see this really steep cliff thing. Drive as close you can to the cliff, then turn of your engine. The chopper will crash into the cliff. Like I said, stupid AI, stupid cops!

No, that's a mission much later in the game. At that time, all you have to do is bring a 50cal turret to the party. And yes, press S to turn off the engine.

I'm playing the nitro pack now, doing the "stampede" missions and doing the one with the bus. I can't seem to do that one. Any ideas?

'Pretty Bus' from the Nitro Pack is fairly hard, but your best bet is a full speed nitrous powered head-on ram to knock the bus to red. Reinforcing your front chassis and using Structo-Bumpers is recommended. Then wander around to kill the bad guys.

I honestly would have never thought of that lol.. I'll give ti a whirl here in a bit.