When TG substitute A340-500 with A340-600 on BKK-JFK route. Here's what they did.
1. Business passengers who are *G or just plain lucky are seated in F cabin
2. The rest of Business passengers are seats in C cabin
3. Premium Eco passengers who are *G are seated in C cabin
4. The rest of C cabin is filled with premium eco passengers
5. The rest of premium eco got a seat next to them blocked, if possible
6. Regular eco passengers are then seated as spread out as possible.
With 44 more seats in Economy section and 8 seats in F, it was possible to re-seat everybody with reasonable comfort. With either op-up to C or have an empty seat next to them, premium eco passengers should be very comfortable.