Is it an old 60's original tank or a modern repro?
I had an original 60s job that looked just like that. The material was incredibly thick, but it was made out of strange stuff, it looked like it had chalk in it! Even before the dreaded ethonal, paint bubbled up as petrol are through it!
Maybe the liner material you used was not good? Maybe it has not covered properly?
Your tank melting into the fuel certainly seems like you're most likely issue.
And TT is totally right about those tea strainers BTW...