What is TFT driver SSD1963 data voltage
If possible, I'd recommend to use 3V3 for the entire circuit; the ATMega is perfectly capable to operate at 3v3. But if you can't for some reason (if you already have a board that depends on 5v to work, or whatever), use the staple for level shifting, the TXB0108. Since it has just 8 lines you'll have to use probably 3 of them (for 18-bit rgb and other signals). TI has a version of this IC with more lines, but is BGA only, which is generally impractical for prototypes.