"desert" tank options

[LAYNFRM]

what are my options if i do the AF conversion on my GEN4 CRF450X frame? since it's a street driven motard, i need as large of tank as i can possibly get. the ones i really want are either the acerbis 4.7 or 6.6 gallor (both store fuel where the radiator shrouds are), but i can guess that neither of those will clear the short necked plug, even with heating. so my real question is, what's the absolute largest capacity tank i can use?

[AngryShawn]

I've seen folks running even the largest tanks on conversion bikes. If there is a will, there's a way.