Even though the water itself works out to just a bit less than 8.5 lbs/gallon, by the time you add in the weight of the tank and any contents, (rocks, decorations, fish, etc.) most aquarists use 10 lbs/gallon to guesstimate the weight on the stand. (150 gallon glass tank can easily weigh 150 lbs by itself)
One thing the OP doesn't seem to be calculating is the bow of the top of the stand. A 2X4 placed so in the "tall config" MIGHT carry the weight without a problem...but it depends on the actual dimensions of the tank. (more than one type of 150 gallon tank)
Personally, I'd be leery of not having a center brace in the front. That's a lot of weight...and a lot of water spanning across the front of the stand.
Everyone who's been in the hobby very long has been indoctrinated into the "wet socks club," but I'd hate to "get my feet wet" with 150 gallons on the floor because the front of the tank sagged and caused the glass to crack...or a seam to fail.