I`m the guy with the 4BT in my `68 Satellite Sport Wagon. I first drove it two years ago, Dec. `09. Have put almost 32,000 miles on it. Ran a 727 with 2:76 gears for 28,000 mi. and then switched to a G 360 5 speed last winter. Fuel mileage is now up to consistent 30mpg, hoping for more. Still using the smallblock torsion bars, just screwed up a little tighter.
I dont know about the added weight of the 6BT in a car without swapping the front suspension to something heavier. And finding room for the oil pan and the radiator will be tough.
On a different note, a little over a year ago I dropped a 6BT/727 in my girlfriends `89 2wd 150 Ramcharger. I used 1 ton front coils and cut 1 1/2 rounds off. It sits the right height and rides nice. 23 mpg everyday.