Balance Problems...Vibration......

A steel balancer won't fit on a cast crank (and vice-versa) will it? I, too, have a vibration, in the driveline, but at 70+ mph. My motor is also a '69/70 steel crank motor. How is balancing handled on these? I've been told a few times, but always seem to forget the difference between the two. I've heard that if you have the wrong balancer, or wrong converter and you would know without a doubt. I think my vibration may be from the holes the front u-joint caps fit into on my slip yoke...I think they are worn and there may be a millimeter of play in them, just enough to cause my driveline to vibrate at speed. Are slip yokes for a 727 available somewhere, or do I need to find a good used one? I remember a few months back when I put new u-joints in, the caps slid in VERY easily on that slip yoke. Sorry, don't mean to hi-jack the thread...just another example of vibration.