Thanks Tophe, at last I've managed to get in here. :)
A P-38 isn't the best candidate for increased span actually. The problem is that the wing planform is markedly tapered already and you can't increase the span much more than what it is before the chord decreases to zero. I ran into a similar problem when increasing the span of my Wellesely BPRIII, and could only add a couple of inches on each side. The Welleseley is a bigger aircraft of course but the P-38 has similarly tapered wings. You'd probably get away with adding an inch on each side but that'd be about it with conventional methods.
On the other hand....
You could change the taper ouboard of the engines, and it wouldn't look too bad perhaps. Changing the overall taper, from the cockpit pod outward, would be quite difficult I'd venture to suggest, but then we're all Master Modellers here of course..... :)