If it locks up tight - thats good.. you’ll have to find out the issue with the underlever.. Loose forend will depend on patent type - may just be a cross key.
450BPE was most popular - if it’s bottlenecked you should see that visually in the chamber.
Yes 450-3.25” NE dies will work - Once fire formed you may never need to use the dies again - check case OAL prior to each reload.
Go buy - have fun and enjoy as life is short.