Why not redesign the whole subframe and incorporate it (the subframe ) into the whole tank, giving you heaps more room for fuel and less worry of clearance issues. Alot more work and engineering , i know.
Certain fuels will swell the tanks and cause the plastic's to enlarge .
This may lead to chopping out, be careful.
Excellant work btw, i hope you get what your after.
Behind the buzzing blindfold of failure sits experience. thats why altzeimers people are always so cheerful.