I will take a beating from the /6 guys for saying this but I would go with a 318 instead of the /6. The 318 can get really close to the same mileage and will have more power with less money. But, if your /6 still runs good then I wouldn't change it now. I would run it and collect the parts as you go for your V8 conversion. Then when the time comes you will be ready.
Chuck