i also think about uprading my 454 mags. the best way if you have good engines is to go for 525 sc. it is easy to mount. a few people in this forum wil help you with the parts and they are very familiar with this kind of upgrade.
maybe the problem is very excessive fuel consuption. how much do you pay for gas in New Zealand. in AUSTRIA the gallon is around 4 dollar. (1 Liter= close to 1 Euro)