Teflon (what pipe thread tape is normally made of) does hold up to gasoline. However, pipe tape is not designed to make a seal on straight threads, only tapered pipe threads.
Silicone (RTV) does NOT hold up to gasoline. Do not use it on carbs, fuels lines or anywhere it will be exposed to aromatic hydrocarbon.
I will take a look at my 76 and 77 fuel tanks this weekend and see if I can offer any helpful suggestions. I suggest biting the bullet and going back to OEM. Everyone I know or have heard of who has tried one of those poly aftermarket tanks has had problems of various types. The most common being fumes from poor seals.