Beautiful welding!
Yes, the radiators are a bit of a pain. Many people have used many combinations. The right side one off the 250 will work fine but the hose connectors will need rotating. It's th left that is the problem.
I bought a new, standard set of KX500 radiators, cut all the brackets off, and started from scratch. This way, I know that I have the right amount of cooling fluid etc. And the left one is not too long. I've also used the standard plumbing as well. I have a 2013 Gasgas 300 enduro ( it's very quick!), and it's exactly the same. ie The rads are plumbed in series. No point in over complicating things...
Looks like a 450 frame you're using?