Fiat 850 gas tanks and engine conversions

[Abarth750 <Abarth750@domain.elided>]

Sorry i sent this early the first time 

I recently pulled the tank out of one of my 850 spider to be cleaned and
sealed.
The problem is twofold. around the fillerneck there are two gaskets. One goes
on top and this is the one you see on top of the car. the other is a foam
material between the top of tank and body of car.  This second gasket has
caused some rust as it has trapped water.
The question is how do i reinstall these gaskets. and two how do i keep
moisture out to keep rust out.

The second part. In another of my 850 spiders, i'm considering putting in a
twincam. I have the motor, and cross member. I'm planning on using a VW
transaxle and a front mounted radiator, ala abarth 1000ot. Any advice from
someone who has tried this.