If you enter the race MId Engine challenge with the 1970 Porsche you win for an earlier race then you start in 8th position. Basically on the very first race Inland Taipei reverse, on easy, you can get the achievement. The other cars are terrible on the tight linch pin curves on this track, so on the very first set you can pretty much bounce your way past the cars. Your car is slower, but right after the linch pins, you can basically gun it with the Porsche, even on the section with red basically through the large C curve, by that time you should have a big enough lead to hold on through the 3 races. I actually won with a margin of over 10 seconds on the next car.
Just make sure you make no adjustments to the Porsche because I dont know if that will affect your starting position. It almost seems the game is kind of stupid and puts the worst car in the worst position giving you a double handicap, on the flip side I have often been in first with best car.