Just for clarification in EASA world you can do the Skills Test for either 330 or 350 (whichever you are current on). Exactly the same as 777/787. You get the other stamped on the same line in all EASA states I am aware of. True there are some differences between state but the jist is meet the 500hr req and do a Skill Test with an EASA TRE in an approved sim.
UK CAA has just released a relevent Information Notice stating they want a license verification by the 3rd country CAA before applying for this service. (effective 1st July 2017). Other states just go with the basics and don't ask any questions. Harmonisation
On the other note deff worth keeping the 320rating ticking over as a backup.