Yes, you can boil longer to hit your intened OG.
You should also be able to calculate how much you have to boil off. The amount over fermentables is constant, only the volume is variable.
V1*G1 = V2*G2
Where
V1 = preboil volume
V2 = postboil volume
G1 = preboil gravity pts (ie, 1.038 = 38 gravity pts)
G2 = postboil gravity pts (ie, 1.056 = 56 gravity pts)
So, if you have 6.5 gals of 1.040 wort pre-boil, and you are shooting for 5.5 gals post boil:
6.5*40 / 5.5 = 47.2727 -> 1.047 post boil gravity.
or
If you want to hit 1.056 OG, you need to boil it down to:
6.5 * 40 / 56 = 4.624 -> 4.624 gallons wll give you 1.056 OG wort.
The above calcuations were copied from a realbeer posting.