If it is a 55 gmc, it will either be a 248, or 270. If you are real lucky someone may have put a 302 in there. Behind the distributer there is a flat spot with a number stamped on it. The first 3 numbers give you the displacement.
There is much to read about swapping a gmc 6 in where a chevy 6 used to be. It always involves moving the radiator forward as the GMC engine is longer. You may have to drop the oil pan and timing cover on both engines and swap the front timing plate. The 55 second series used a funky horse shoe type of motor mount that bolted to the side of the engine. Use your original bellhousing and clutch linkage from your 53. You can put the larger 11 inch clutch setup on the gmc flywheel... an upgrade from the 9 1/8 clutch on the chevy.
John
