Something like a Mavic Open Pro 36H would be a good rim to use.
An Open Pro is way too light IMO. At the OP's weight I would get something well over 500g, and probably look for something with a depth of about 30mm. 36 spokes on the rear for sure.
If you weigh 270lbs, does it really make sense to try and save 1/2 pound in your rims?