As was said you will lose a lot of towing and hill pulling power, but cruising and highway milage on the flats will be great. I have 3.08s in my 84 chevy c-10 with a 305 and 700-R4 overdrive and it gets pretty good milage for a v8. Actually does fine in hills too, and handles weight well as long as it is in the vehicle and not towed behind it. I run 31x10.5" on it. A better idea in my opnion would be a 700-R4 swap, as it will allow you to have the best of both worlds. For the record if I keep my truck Im installing a 3.73 ratio with a limited slip differential. Milage will drop a lil bit but acclleration and towing, and weight capabilities will be more than enough reward.
Good luck and have fun--ybi
