I was there - heading north - in September 2003 and there was no fuel on the Mozambiquan side - I gave some to the border guards as I still had 30+ litres. you should be fine with 300km range, but the more the merrier as it was often hard to find in Mozambique. there's plenty at Lindi on the Tanzanian side, but then little till nearer Dar.
back then, the road was dusty and broken. stretches of tar near Lindi, Kilwa and 100km south of Dar, but dust etc before and after. that said, they were surveying and I was told the road would be all-weather by the end of 2004 so, by now, it may be really boring :-(
if the pirogues and dhows are still running across the Rovuma, its much nicer than a 10t ferry! have fun.