Hi Mike,
I know the owners manuals tell you to use the fibreglass bodywork for jacking up the car, but personally I prefer not to.
At the front I normally place the jack under the central cross member and lift both front wheels. At the rear I normally lift off the rear chassis loop (where the gearbox end mounts are) by using a wooden spacer to make sure the jack doesn't slip. If I just want to raise one wheel then I put the jack under the box section trailing link, as close to the wheel as possible.
The horizontal fibreglass directly under the tanks is only thin, about 1/8", and completely seals off the tanks from the road making a proper compartment for the tank. To replace either petrol tank I think the only way is to cut out these sections, lift the car & drop the tanks out from underneath. (at least that's the way I replaced mine)
I used 2 sheets of thin aluminium pop-riveted in place over the holes rather than replacing the fibreglass; if I ever need to do anything with the fuel lines then it's drill out the rivets for much better access.