I'm confused about the question. Doesn't a 3800 already have an open driveline? I know some early 3/4 tons had a torque tube to a carrier bearing and then an open driveshaft back to the rear. I am nearly certain that all 1 ton pickups had an open driveline from at least 1947 on.
To answer your question, yes a car or later truck 3 speed will fit. Do you have a 3 on the tree? Many times the linkage can be modified to work. A better choice would be to replace the 3 speed with a 4 speed or even 5 speed. The transmission floor cover would likely need to have a hump if you went that route.
Something still is making sence to me about this post. Is this a 3600 or a 3800? It makes it difficult to give you a complete and accurate answer when I don't know if your truck is a 3/4 ton, 1 ton, or other.
