I used a new tank from Andy on my 68 Trade Wind. If you buy a fiberglass kit from Home Depot or even Auto Zone, it will include resin and glass cloth. By all means use the cloth. Just cut strips and lay them around the opening. Wet out the area around the flange and vent with resin then lay the cloth on top and apply a little more resin to fully saturate the cloth. A small disposable paint brush works well. Rough up the gel coat around the hole with some sand paper before you glass the fittings to give the new glass a better surface to bite. If you have any doubts about how, call Andy he will tell you.
A couple of things to consider before you cut the hole. I nearly made a big mistake in positioning the flange on my new tank by just measuring the old one. I would highly recommend that before you cut the new hole that you mount the tank under the trailer. Get is exactly where is needs to be and then go inside the bathroom and mark the top of the tank through the hole in the floor and do the same for the vent. This will give you a perfect match. Don't just measure the old one. Take care cutting the hole as you want the flange to fit just right before you glass it in.
Bruce & Rachel
68 Trade Wind
2001 Toyota Tundra