4BT Cummins in a full size pickup, a good idea or bad one?

[WyattTX]

As we all know 6BT/12 valve swaps are common and seem to have good results in performance. But I have thought about building a truck for fuel economy, rather than raw power. I hear of people getting 25 or 30 miles to the gallon with a 4BT Cummins. Is this really possible in a full size pickup such as a 67-72, or 73-87? If it is possible, how much would you be sacrificing to get that kind of mileage? I would hate to end up with something super sluggish or weak. I know little about the 4BT, so I am interested to hear some real world results. Thanks.