The problem is the holes in the fibreglass have to be quite big to locate the tank, and they seem to get bigger with age.
I have had to make up a few quite big plates to spread the load where the holes have become too big.
Normally they have "penny" washers, about 30mm in diameter.
Actually getting the bolts into the tank is a "challenge, but once you have 2 in, the next 2 are easier to locate.
You will need a lot of patience to get them in, and beware, your arms are likely to get rubbed against the fibreglass, quite badly.
Use a ratchet spanner to tighten the nuts.