I’m a relative newbie myself but have been digging deep into post types, and restricting (many) things within Admin. I believe what you are asking for is not something you’ll simply configure; it’ll require code:
(1) Add a metabox to your post type, with your drop-down title choices as one of the fields in the metabox.
(2) Turn off the edit-ability of the #title element on the post page:
<style type="text/javascript">
jQuery(document).ready(function() {
jQuery( "#title" ).attr( "disabled", true ) });
</script>
enqueue that .js, or just inline (echo) it at admin_footer hook – test for your post_type to avoid inadvertent shutting off #title on other pages in admin.
…you may also have to stick a temp value in the #title element, to get the post to save; I’m not sure if that is a required field for saving a post.
(3) Hook into action post_updated, check for your post_type and copy the value of the metabox setting (your desired title) into the post title field, via wp_update_post($post) function.
Obviously there’s a ton of detail to be filled in, but that’s the way I’d approach it.
(edit): OR – do the whole thing with jQuery; completely re-write the #title element on the post edit page, so the title field becomes a drop-down selector…