Ginge....
No prob mate
if it did need top rails i would,ve put the info in the thread discussing the rear tub,
basically the tub is secured with the new square tubing as mentioned on the other thread & where the tub meets D11 & SB1 tubes & i think its D9/10 (haven,t got my book at home)
and yes Adrian H is correct,,,,, the "book" dimensioned fuel tank won,t fit with a GRP tub, tho once the tub is trial fitted you can then get measurements to fabricate a tank, or do what i did & bought one that fits
cheers
andy