I have been doing restoration paint jobs for years and have painted in pieces (at different times) for most of that time without having a problem with different shades. As long as you buy enough paint for the entire job in the beginning, there should be no problem with the color matching (especially on a solid color like red).
The last Corvette I painted was a light metallic blue and I painted it piece by piece at different times then assembled it. Everything matched perfectly. On a metallic job (which shouldn't affect you any), it is important to at least have the panel sitting in the position it would on the vehicle (don't lay a door flat, but have it stand up like it would sit on the car). Metallic stands up differently on panels laying flat compared to panels standing up and severely affects color.
Anyway, I would use a good single stage urethane (no clear coat) for a solid color red paint job. It will cost more than the acrylic enamel, but is much more forgiving and will ultimately give a much better finish and will be easier to color sand and rub if needed. It sounds like you are going to have a very nice fire truck when you get finished. Good luck!
