I think the 292 L6 would be an ideal replacement. It's a very well-built, 7 main bearing engine of modern design. It was used from 1963 through the late 80s in Chevrolet (and some GMC) pickups through 60 series medium duties. It has outstanding torque and reasonably good economy. It is not a bolt-in for the 270 however, as it uses the Chevy V8 bellhousing pattern. The 292 is still reasonably plentiful and I see them on ebay regularly. They were the 'standard' engine on UPS and bread trucks for many years before the diesel replaced them.