The 46.5 is made because when you add the endcaps you get 48" total. If you buy 48" bulbs and put the endcaps on you will get 49.5 inches. Measure your inside hood to see which would fit better, both are 110 watts, if both fit go for the 48" if not use the 46.5".
Ah I looked again and you are building your own hood. OK if you are going after a 2 bulb kit you could angle them to fit a 48 inch hood if you are going after the 4 bulb kit then just build your hood a little larger for the 48" bulbs. In this case I would go for the 48 " bulbs because in an emergancy and you need bulbs now they will be easier to get locally instead of trying to order a 46.5" bulb and having to wait.
Just my 2 cents
Thomas