Hey there. With the top are you removing the entire thing or creating an edge frame for your fixture to rest on? Vivosun fixtures have some weight to them. So you want them to sit on the frame not straight on the screen. So this will help when it comes to which fixture your picking as well.
So with the fixtures. The fixture length for a 3 foot is 36 inches. The bulbs are 2 inches shorter so 34inches. So then a 48 inch fixture would have 46 inch bulbs. So if cage width is 41 with a 48 inch fixture centered your going to have a 5 inch over hang 2.5inch on each side with the bulb portion. 3.5 inch each side total fixture. This is where it matters for your cage size... Your 41inch with a 36 inch fixture means 34 inches of light. so you have 7 inches of an area where you do not get light. To me this is more area left uncovered than I would prefer.
I ran into this with my cage. It is a 30 inch wide. I chose to go with the 36 inch fixtures. I have them centered so that my total overhang is 3 inches on each side with 2 inches being the bulb. I would have lost far too much light putting a 24 inch fixture on with 22 inch bulbs.
This fixture does have separate reflectors so you want the 6% bulb and you want to place that in the position most middle of the cage. Typically people will pull a vivosun to the front of a cage with about a 2 inch gap in the front of it to create a vent space on top. Then have basking in the back corner. So Uvb would be closest to basking.