Ok, great! Then it looks like you're on your toes and know what to do. Personally, I wouldn't wait until you get to SI. The Columbians are there and often enough would blow the Ambulance up. It's best to just get it out of the way in Portland. When I was younger in 2007 playing this for the first time with a friend, it took me and him like 5-6 hours consecutively of trying to pass this. However, when I did this just a couple months ago, I got it 4th try. The trick is to stay calm, but go as quickly as you can. The time limit is also pretty strange. Sometimes, upon reloading and retrying, the time would increase or decrease, and when I was getting nice times, I continued and finally passed it. I did Level 4, Leve 4, Level 11, and Level 12 on my 4th try.
BTW, are you also going for special vehicles that have special properties like bulletproofing, damageproofing, fireproof, and explosionproofing? Cuz if so, 3 proof vehicles can be obtained in Portland, BP/FP Securicar is first, AP/EC Stretch is second, and BP/FP Trashmaster is third. There is 4 proof vehicles in Staunton Island, BP Barracks OL, BP/FP/EP Bobcat, EP Securicar, and BP Patriot. I saved the BP/FP/EP Cheetah for last, since the Turismo mission in Portland is always active, even after 100% and it's much easier to use a Tank than a Fire Truck to get that vehicle, imo.
Best of all, you can use the Crusher Glitch to make the Bobcat and Cheetah DP, which will turn them into AP vehicles like the Stretch, and you can also make the BP Patriot and BP Barracks OL DP, which will convert them to BP/DP vehicles. It's very fun collecting proof vehicles.