If I'm guestimating right, 160 gal of fuel will be 128 lbs heavier and the engines will be ~100 lbs heavier (SR305-230) for a total of ~360lbs.
Now the Ravin 500 twin (airframe) would have a lower empty weight due to the fact that it's composite, that would off-set some of the difference. I'm sure it wouldn't be too hard to increase the gross weight by a couple hundred pounds since we're talking about an experimental that could (I hope) be built from plans. Slightly beefier landing gear and a slight increase in wingspan should do the trick.