I own a driveline shop here in So Cal, and what you are describing sounds like a classic driveline issue.
What series is the chassis? If it is a 3/4 ton, then you would likely need a hybrid u-joint to attach a T350 yoke to the existing shaft, and there are something like 7 or 8 different types of 350 slip.
Also, you say all you had to do was put the slip on? I find it highly unusual that you did not need to modify the shaft.
Something else to look at engine wise, are you sure you have the right flexplate? The 400 is an externally balanced engine, all other smallblocks are internally balanced. Put the wrong flexplate/flywheel on and you got yerself a 4 wheeled paint shaker. Don't ask me how I know.
