A guy is selling his takeoffs from 2015 softail. What would I need to make them fit my 2000 Heritage?

 

3/4 inch wheel bearings. Possibly respacing but maybe not.

All ol I can think of unless it's a 200mm rear, then you're in for a whole list of things.

 

I did a thread on this exact thing in the wheels and tire section. Spoked wheels. The later model rear hub is wider. A lot wider. The front was fairly easy......at least for me. I have a lathe and mill, and I made custom sleeve spacers for the front. Done.
The rear, I unlaced the wheel, machined .400" off the pulley side of the hub, and bored the bearing holes deeper to use the wide 3/4" bearings. So that now fits like a stock wheel.
Not to complex on a spoked wheel. I now have the late model tubeless smoothies.
A one piece aluminum wheel could be done the same way if one had a lathe that could swing the entire wheel.