I ran a 390 with .060 overbore, 10:1 flat tops, and a 260/270 rv cam, with the relatively stock 2bbl carb and intake(gasket match ported heads and intake manifold, along with mild reworking of carb jets and metering rods for side-hilling) and I never pulled a gear i the mountains. I lived in Jasper, Alberta for 8 yrs, and had that truck maxed out for towing and hauling a few times and all 4th required was a bit more fuel to keep it at speed up the hills. If your engine is stock, you may suffer. With everything I did, I was running an average of 18 mpg highway(empty) and around 13-15 loaded, in town was ok at around 10-12 depending on my heavy foot. Mileage would have been better with an intake and off-road carb added to it.