I had to do that to my 55 gal a few years back when it was a fw tank.It is a pain,but worth the peace of mind to have fixed,and cheaper than a new tank.
I bought the box,and I did not like it. It is hard to keep the proper water level to make it work properly. Also,it took up a lot of room in my 55 gal tank. Wish you lived close by,I would sell you mine cheap.