Im not a huge truck/car person have two transmissions that came of off two different second gens both are 24 valve 5.9L one transmission is stamped 238-4 and the other is 238-3 which is apparently a built transmission, but they can’t be swapped between the two the 238-3 on the inside where the spline is, is shorter causing about a 1/2 inch gap between where it mounts to the engine. Did I get sold the wrong transmission or do I need to get a different torque converter? literally can’t find anything online

 

They should be exactly the same..... Where is the gap you are talking about? Engine to trans mount? If so, probably the torque converter isn't completely seated.