If you are talking about a 40 breeder, I personally wouldn't use the 1/8" glass. You can go to a local glass shop and get 1/4" glass for less than you will pay for a conversion kit. I got 1/4 " glass to make full lids for a 40b and a 70b and I think I payed less than $40. (It was a while ago)
I personally don't like the typical conversion kits, however with kids around, the trim might protect the glass a little, the edge is the weakest.
. I have been making mine a little differently lately, I remove the trim and sometimes the top piece (depending on what will be living in the tank), then do the conversion. Afterwards I modify theold trim or buy new to put top and bottom on and then put a piece of glass in the trim (just like a lid on a standard tank). When finished, the tank looks like it was manufactured that way.
Sorry fir the minor hijack.