I can't help you much because I haven't used it. But from what I've read about them, you have the correct size of tank for the mega ray. They are the best MVB's you can get. Bob Mac, the maker of these bulbs tests every bulb, so he knows they are all safe.
I too have the 40, And have the 36in 10.0s. If the megaray is on backorder the next most bang for your buck it the? Zoo med mvb or the T rex? And duration time. Oh an sorry to jump in on this.... :?
check out the external ballast mega rays, they cost a little more but well worth it in my opinion. I have 2 enclosures(4x2x2) w/SB100's, the temp are hard to keep resonable, basking temp of 116* @ 14" & a cool side of 95*+ . I've had to remove the glass doors on the coolend & install sliding screens to get it down to 83*-85*. I recently aquiered a young dragon & set her enclosure(4x2x2) up with a EB 60. Along side of the EB 60 I have mounted a 100 flood light for heat. I put the flood/heat light on a dimmer to adjust the temp to right where it needs to be.