If you would like to keep your truck a 6 cylinder, there are many available that are direct bolt in replacements for your engine. I assume your motor is a 228 6 cylinder. There is a flat pad on the engine where the distributor goes into the block. There should be a motor serial number stamped on the pad. The first 3 numbers of the motor serial number are the engine displacement. There also might be an issue with 6 volts vs 12 volts.

If your truck has the GMC 6 you could use any 1939-1953 228, 1939-1956 248, 1941-1959 270, or 1951-1959 302. 1954 and later engines might pose some problems due to a different front motor mount setup. Additionally some gmc motors have 4 lug flywheels while others have 6 lug flywheels. The flywheels from bigger trucks are different than the flywheels from the 1/2 ton through 1 ton trucks.