I agree the overdrive tranny will be the trick set-up,and with the right gearing you can get pretty good mileage with your bigblock.But it IS possible to over-gear your truck. If you run the 700R4,you might want to stay close to a 3.55 or so rear ratio. You can fine tune your final drive ratio a bit by changing tire size. Otherwise,you could spend a lot of time in the reduction gears or with the pedal floored trying to maintain road speed. ( Ask anyone with a late model,tall geared 4X4 with big tires)At least with a big block,it'll cope with too much gearing better than a smallblock.
BTw-a guy I know drives a '77 Nova with a 700R4,2.73 posi rear end and a built Caddy 500.It just flat HAULS A$$,and gets 19 to 23 mpg on the highway.
