I make at least 1 trip from AZ to IL a year and usually rack up 4000 miles by the time I get back. I travel mostly interstates running 75-80 MPH the whole trip. I've done it once in the wife's 84 El Camino (1973 LT1 350/TH350/stock 10 bolt with 2.70 something gears and 225/70/14 tires) and at lest 6 times in my 83 El Camino (507 Cadillac/TH400/Ford 9" w/2.75 Gears and 235/70/15 tires).
Out here in AZ most of our secondary roads are 65 MPH and Interstates are 75 MPH and I do very little in town driving so I geared accordingly (and the 507 Caddy doesn't need a lot of gear anyway). Gas mileage with either isn't phenomenal; about 16 MPG with the 350/350 Combo, and 17-18 with the 507/400 combo, at 75 with the AC on. I live at 4500 ft above sea level and where I travel to is at 200 ft so I usually step up to premium fuel on the trip and take a couple of degrees of timing out of the engine.
With your drive train I wouldn't expect great mileage, but you should make it there and back with no problems..........the one suggestion I would make is to be sure the cooling system (especially the radiator) is in good shape.