I don't think I'd try it. Most importantly, the internal dimensions of the stand are likely going to be slightly smaller than the 72" in length. For a stand that size there is likely a couple of braces that you will need to remove to fit the 150 inside - which isn't a problem as long as you do it before you put water in the tank and can reconnect them without compromising the weight-bearing integrity of the stand. I'd also check with marineland to find out the height of the doors on the stand - you'll want to make sure what you have enough clearence to get equipment in and out of the sump (such as the skimmer collection cup, etc) and room to work inside the stand to make plumbing adjustments, move things around in your sump, etc - esp. important if you are going with a 150 tall. Finally, that's going to be a lot of weight for the stand to support - not only the filled weight of the 300, but also at least half filled weight of the 150 plus equipment, live rock, etc. I'd definitely find out from marineland whether the stand can support that much weight, and whether that area of your floor can support that much weight!