Hey mate,
I've got a zuk ute aswell but mines a Maruti (indian mate zuk) LWB. Its still currently got the Maruti 1lt in it but will soon be running a 7kc.
3400rpm @ 100kmh doesn't sound too bad at all, but this may be due to your 31's.....and the 5speed. Mines only a 4speed so @ 80kmh I reckon the 1lt would be doing over 4000prm. I can get 100kmh out of it but that would be more like 5000rpm so you don't want to hold it there too long.
As for fuel consumption I'd get about 300km out of the 40lt, so for the sake of having a motor 1.5 times the size and only getting 50km less from the tank I'd again not be too worried.
Hope that helps.