I have 20"X10" 295/40/20's and there about a 1/2 " or more larger in diameter. I put a set of 17's from an 01 GT over my wheels to measure this difference and it was considerable considering the series I'm running.
Holy cow dude Those are 29.5" tires!!!!! Stock 18"s are 27.2". Have any pics of your car with that tire/wheel combo?!?!?!?!?!
Your 40 series tire doesn't mean it should be lower profile than the 18" stock 50 series. The series # is % of the tread width. 40% of 295 is 118 mm. 50% of 235 is 117.5 mm. So that means your 20" sidewalls are the same height as the factory 18" sidewalls, but you have the added 2" in diameter wheel that gets added to that. A 255/35-20 would give you nearly the same overall diameter.
I originally had 20 X 8.5's in the rear 255/35/20's like you mentioned earlier but I swapped them out for the 20 X 10's. I only have pics with the 8.5's but I will take some pics of the car tomorrow and post them. I've had trouble trying to post pics because I dont know how to shrink the pics I take with my digital camera. They are always huge. I will try to get a clear pic of the dyno chart and post that as well. Later fellas