Red test lead will always go on whatever the positive battery cable goes to, in your case it will go to the chassis. If you reverse the test leads it won't cause any harm, the meter will just read in negative voltage, unless you are using an analog meter in which case it might peg the meter at the zero end and you could end up with a calibration issue.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy