ERTF,
You may want to check out this TF thread from a year or so ago:
My husband and I have noticed a small pooling of diesel fuel accumulating in front of the fuel tanks, between the outboard hull stringer and the hull. We have investigated, and so has our local boat yard, but we have not been able to find the source of the leak. We are speculating, based on what...
www.trawlerforum.com
The OP had a GB36 with fuel tank leaks, but the post you should read is from Calypso, who replaced the tanks on his Island Gypsy, also and older model, and also with the tanks outboard of the engines. He did it from the outside of the hull, and provides a lot of detail of exactly how he did it.
It may be more doable than you have been thinking, and to me, at least, have new but like-original tanks will serve you better over the long term than plastic drums, etc.
I was looking at a boat very similar to yours, but eventually decided the logistics of doing everything from the opposite coast made it too difficult and costly. But that is a really nice boat, and your peace-of-mind will make it worth doing the job right for however long you keep the boat, and should also make it easier to sell if you ever get to that point.
Good luck, whatever you decide.
Peter