Sounds stupid, but......

What is a good way to tweak a bent chassis?

I have a Fujimi F430 Challenge that I dry fitted last night and the flat chassis is warped. I have not come across this before because I only build Tamiya (this is why). Any ideas from you all out there?

I have run across this before (with a Revell Corvette, the one in this forum). First, make sure that the chassis does not "unwarp" when you put the interior tub on. Tape the tub firmly to the chassis with masking tape, which will replicate how it will sit when glued. If the chassis sits flat, you're good to go. Just use plenty of glue when assembling and keep it taped together until the glue has dried thoroughly. (I recommend ProWeld glue by Ambroid for this; just paint it into the seam and it is drawn in through capillary action). If this doesn't work, however, there are two other options.
1. Twist the chassis by hand. Obviously, don't break it, but give it a firm twist in the opposite direction as it is warped. Repeat a couple of times, until the results are satisfactory. The plastic will bend to some degree.
2. If this doesn't work, try a little heat. The best heat source I can think of is a powerful lamp or a heat lamp. Hold the chassis close to the lamp for about 10 seconds--be very careful, because the last time I did this with a '59 Impala hood, I ruined it. Don't hold it too close. Once heated, gently bend it in the direction you want it to go, then preferably dunk it in cold water to keep it from either bending back or bending more in an undesirable way. Repeat if needed; just be careful. Hope these work for you! The F430 is a great car; I'm sure your model will look awesome.

+1 what he said, but use hot water to heat.

Assuming your hot water heater isn't set to a million, just run a bucket or sinkful of hot tap water, then dunk and twist it by hand until it looks right. Dry and re-test-fit. Rinse. Repeat.

Great, thank you all for the ideas, i will try asap!