your mpg won't suck too bad with a 6bt... we have one in a corvette at work that we run on straight veggie oil! check out greasecar.com
you can pretty much make everything you need, except the computer controller they have, which you don't really need.
i vote 6bt!! i'm not a huge fan of the chevys or fords, because i've driven vehicles with all three, and i like the torque and whatnot of the straight six. there is a lot you can do with the older style 12 valve motors....
buckle up, i wanna try somethin...