Try and avoid cutting it in half, the join can do funny things to mips and it's not fun to paint. Problem is I seem to remember there was an issue with not-square textures and bumpmaps? in that case I guess as long as all the parts are exactly the same size and hopefully mapped in divisions of two or four from the edges - so if there's any compression involved it won't make one half look different to the other - then we can cope. One thing I really hate though is when people map little random parts way off in some unrelated corner of the texture and you end up spending a whole day looking for a couple of nuts. One other thing I hate is when you reference textures in the default folder too. And if you can get the sides and the ends ( and the cab ends ) all the same scale, that helps.
I rather like Derek's latest unwraps, and actually the default ES44/F7 aren't bad either.