Grampa gave me his truck

[sirjosephanybodyxiii]

My grandfather recently gave me his 1992 chevrolet 1500 with a 6.2 liter diesel engine. He bought it new with just two miles on it. It now has just about 400000 miles on it. After spending time fixing brake lines and wheel cylinders I finally drove it only to realize something is wrong with the drive train. The truck reaches thirty mph and then the engine races and won’t change gears until I floor it. It does this every gear change until 65mph where it goes no faster. My grandpa did a lot to this truck to make it fast so it doesn’t take long to get there. Anyway I was thinking its either the transmission or the torque converter. My father in law also has a 1989 chevy 2500 with a blown gasoline engine and said I can take any parts from it that will fit. So if it is either of those things will I be able to transfer the parts from the2500