Weren't the clodbuster guys making D cell packs like 10 or so years ago??? There's also a chassis out there which will let you use 4 sub C packs...

Heat is definitely an issue to be considered, there are ways of combatting it... Long before the packs we have now, myself and a few others I raced with were freezing our packs... I'd show up to the races with my packs in a cooler... It always seemed to run a lot stronger and the packs were a lot cooler at the end of a race... I wish I had the batteries then that I do now, tryng to make it to the end of a 5 minute modified race was always a chore...