I think the only thing I'd be worried about is side to side play in those stands.. have you given the car a good shake from the side? does it wobble any? if not I'd say you are ok.
Only thing though.. much did you spend on wood? I've got these jackstands (3 ton high lift ones) and its plenty high enough to get my big *** under the car.. 25 bux a pair.
http://www.sears.com/shc/s/p_10153_1...&blockType=G11