Some additional reasons for not extending the fuel tank are:
    *will cause you to fly heavier (less baggage, less climb and speed) on
each flight unless you standardly intend to                           leave
extended tanks unfilled
    *will cause more aft cg
    *will cause more frequent flight at over gross weight conditions
    *will cause off-loading of fuel much more frequently to achieve proper
cg and weight
    *more time
    *more money and effort
    *will screw up your paint job
    *may induce you to attempt longer flights which is often not safe (fewer
rest breaks)
    *will exceed the bladder capacities of both you and your passengers

My guess is that it will cost you at least hundreds (including paint) and
will take months, not weeks to complete.