For Delta you will either need an American Express card earning Membership Rewards or points in a hotel partner's loyalty program to transfer to SkyMiles. Note that Delta themselves do not have a First Class cabin for long haul international flights, as those will be in Delta One which is business class.
From Chase URs, you do have several options for Delta flights:
1. Use your URs to book in the Chase Travel portal with a 25% bonus. i.e. for your international business class ticket, you might only need 800,000 URs rather than 1,000,000 URs to book the ticket. (If you are going this route, it would be a waste not to get the CSR for the 50% bonus as you'd be saving yourself about $2000 in URs just on this one example ticket).
2. Move points to Marriott in increments of 60k and then transfer to Delta at a 2.4 to 1 ratio (i.e. 480,000 URs would become 200,000 SkyMiles). Outside of fare sales, this will likely result in fairly poor value, but sales like 80-120k for Delta One to Asia or Europe have happened in recent years and this would be a great value.
3. Move your points to Air France/KLM or Virgin Atlantic to book Delta flights. This will generally be the best value. AF/KL can be chaotic and all over the place though, so plan to look for alternate dates and routes. Virgin will be a better value the more direct the flight is.
