I would paint as many pieces one at a time as you can, you will get better coverage with the smaller sections and be able to protect edges and laps that would otherwise be bare steel if it is assembled. Save the final color coats for the assembled project.

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