600 centres refers to the distance between the centres of the joists, not the gap between them. The boards are 2.4m long to accomodate this spacing - with wider or narrower spacing you won't be able to have all your joints resting on a joist, and that is the most important factor in determining the strength of the floor - you could break off an edge of a board but you won't go through the centre of one.

I'd also want to think about the weight pressing on the steel roof truss because it is designed to be held under stress, not compression. (Or is it the other way around?)

For bumpers, trim bits etc I don't think you would have much to worry about but engines, gearboxes, lots of starter motors/alternators/struts I'd have another think about things.