Not an engineer, but I would guess all other things being equal you might have 10-20% less fuel consumption with a V6 as opposed to a V8, if you didn't drive the V6 very hard trying to equal the performance you would have had with the V8.
One thing you might want to keep in mind is that GT40s don't accumulate a lot of miles, even in the hands of the most enthusiastic owners. Most of the time you need to drive somewhere, the GT40 isn't practical, so you will end up driving something else with more space (and less fun). Total fuel comsumption over, say, a year may not be enough to worry about the difference between a six or an eight.
Now, there may be engine size taxes or something of that sort in Chile that we aren't aware of up here and that could favor a smaller engine. A GT40 is light enough that a robust V6 should still give enough performance that you can scare yourself.