I have a couple of REO's 331 gas engines. They were in the same era of the GMC 302s. They were used in the M35 6x6 and civiliar applications. Very nice engines. Gerotor oil pump, wet sleeve, water manifold, 7 main bearings, one piece rear seal. The civiliar version had an oil to water cooler. Either can be used with a full flow oil filter. Only real drawback is that it uses an integral manifold and the ONE intake port is rather small. Parts still available, but the engine is longer (head is 32" long) and heavier than the 302 gmc at ~800-900lbs. Hate to say it but a better engine that its competitor, GMC.
