I have an 88 k1500 350 4x4, 8th vin character is “k” and I wanna know what and if I can swap out that motor for a duramax and which one would be the best one to do so and do I have to swap out tranny as well?

 

You realize that you will be putting in a diesel motor that is turbocharged & Intercooled I hope. Likely have to change the transmission out to an Allison automatic. You are talking about some serious money here. I would trade up to what you want, not have all that work done on a 33-year-old vehicle.