In my current game save I have been playing around in just the Beginner League trying to figure out which car I prefer for starter car. The Trueno has been one of those cars I've spent a lot of time with. Having done so, here are some of my thoughts that might be helpful:
Upgrading the Trueno:
Because the Trueno gearing is so high, running the automatic transmission with stock transmission is a hindrance. This car/series is the perfect venue to teach yourself manual transmission (specially since you're racing it and loosing anyway... one more time won't hurt

Just joking). I would highly recommend it. The Trueno is like a different car with manual transmission... really. You have waaaayyyy more control.
I would also recommend that you purchase the full racing tranny to help you along before purchasing the stage 2. Especially if you are staying with automatic.
I have found that for me, softer is better on the suspension of the Trueno. It breaks loose too easily when it is stiff. In this series, at the HP you are talking about, I actually found better performance with the stock suspension and I didn't run any tire upgrades.
Driving the Trueno:
At all costs, brake in a straight line with the Trueno. Practicing careful throttle management/feathering through the corners is critical. This is where the manual tranny shines guys. Keeping the car in a gear that keeps you at the top end of the RPM scale and feathering the throttle through the corners to keep the tail in check with no sudden moves is where it is at. Do this and the Trueno will cruise through the corners with momentum to build acceleration from out the other side. Don't do this and the Trueno will loose sooooo much momentum it is a month of Sunday's getting it back up to speed.
I know this is generic but is especially true with cars like the Trueno. You need to manage your race much more. Track placement and line are crucial because the Trueno sucks so bad at acceleration and so easily looses momentum. Clean apexing and never letting the AI be inside you are absolutes. You so much as get bumped out or swing the tail and you will scrub 20 mph off your speed in miliseconds. You are better off slowing in controlled fashion and letting the AI go by than to push the line and have them bump you in the Trueno.
In short, if you drive the Trueno nearly in the
exact same way on a dry track as you have to drive it in the wet license test and you will win this race series easily. It is all about momentum management.
phattboy