It likely is a GMC 302 engine bored 0.030 over with a gmc 248 crankshaft for a bore of 4.030 and stroke of 3.8125. This would shorten the stroke to produce a higher revving engine.
A gmc 270 would have to be overbored more than 0.150 to reach 292 with the standard 4 inch stroke.
270 gmc standard bore = 3.78125
270 gmc overbored to 3.935 = 291.87 inches
