Eddie,
I have a keel tank just as you on one of my Allmand 35's. The former owner reported it leaked so he had bypassed that tank. I have notice what appears to be an access cover above that tank that is fairly large. If yours doesn't have one I would make one. I have opened up other fiberglass tanks and to repair them from the inside you need working room as well as being able to get grinders inside the tank and enough room to use them.
On the 31 ( I haven't looked at the 35's yet) the factory installed a very large cover. It is difficult to remove after the screws have been pulled out because they used a very aggressive sealant but it is possible with care. I have done it. On my 31/34 I turned the 50 gallon tank into a 110 gallon tank by jigsawing the vertical seams apart and moving the tank sides outward, forward downward and aft as the case may be for each side. Even at that I still left some air space between the tank wall and the hull side. It has worked great for the last 29 years.
Relative to your suggestion that you would use epoxy I have some observations. Our fiberglass laminates Do Not include any epoxy. That material excels at needs that require adhesion or if it was used originally in the laminate for strength or weight savings. Polyester or Vinylester resins do not stick well to epoxy. Only epoxy sticks to epoxy. When I do all my fiberglass work I stick to the two fiberglass resins mentioned. You are never going to make the boat any stronger or lighter by using epoxy. It only complicates future repairs. Take the time to layup the fiberglass correctly and get any excess resin And air out of the laminate by using a fiberglass serrated roller designed for that purpose. I like using one of the smaller versions for jobs such as yours. I have had great success with this method. It also reduces any fairing necessary if you have rolled it properly with those rollers. My more than 10 foot wide transom has survived 31 years, thousands of offshore miles for 4 Cape Hatteras roundings, one of which was quite noteworthy.
Best of luck with your repair,
Keith on several Camelot's