1981 KZ 1000 LTD trees interchangeable with 1978 KZ 1000 LTD?

You CAN NOT spread the triple clamps to fit larger diameter forks, you risk the clamps failing and ending in disaster. It sounds like you have no idea what you are doing, the front end is designed around the frame geometry, simply bolting, or trying to bolt on foreign parts is not the way to do it, unless you have some understanding of suspension geometry.
Rebuild your original forks.....

The upper and lower triple do clamps the fork legs by form (max. clamping surface) and pressure. You can do a +1,00 mm over size bore but this can weaknesses the triples.
Do not play any games with you health.....

Thank you, everyone, it was just a thought.