Put the battery and tank in either corner of the trunk (nearest the taillights). That space is useless anyways...you'd be surprised how much crap you can fit in the trunk and still have usable space!!

1999 Silver Frost SVT #609 of 2760 Quaife, lightened SVT Flywheel, SPEC stage II clutch, removed resonator, k&n drop in - various other goodies too.