1st: Bigger flies better. A .60-size trainer will be easier to see in the air and will generally be more stable, but there isn't a huge difference.

2nd: You can put the wing in the truck bed diagonally, so a 6.5' or 7' wing might still fit.

3rd: Wing bolts can be just as big of pain or worse than rubber bands. Disregard this as a decision factor, rubber bands aren't bad and wing bolts aren't that great.

4th: There's no reason why your K&B .40 can't fly any of the commonly available trainer ARFs. I'd say go ahead and use it.

5th: Save the Saito FA62a for the Pulse XT 40 ARF. It would power a .40 or .60-size trainer just fine, but you'd need to rework the throttle linkage a fair bit. You already have the K&B .40, and there isn't much point to putting a $200 4-stroke at risk while you learn the basics. The four stroke would give you longer flight times due to better fuel economy, however.

The plane you want is the World Models Sky Raider Mach I trainer:

http://ecsvr.com/abm/shopexd.asp?id=2925

This $99.99 trainer ARF has a nice big 65" wingspan (1" shorter than your truck bed) and a bolt-on wing. It's well made and comes with good hardware, but the instructions may be a bit sparse if you haven't built an ARF before. It would fly very well with your K&B .40 engine.

Good luck and good shopping!