Your PSU looks great.
The maximum safe temperature for a 680 is 98C.
http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-680/specificationsThe cards also auto-throttle and boost depending on their temperature and other factors such as power draw.
The cards are designed to work with multiple of them side by side and still be cool. So your power supply just being close to the cards should be fine.
I have two ASUS GeForce DC2 670s in SLI which are rated at a maximum temperature of 97C. Only under heavy stress (Heaven benchmark) testing the top card reaches 73C. Under normal gameplay (Battlefield 3 Multiplayer) the temperature is lower. My case has four intake fans and two exhaust fans for comparsion.
I think you will achieve similar results with your 680s and will be fine.
CPU: AMD Ryzen 9 5950x | Mobo: ASUS ROG Crosshair VIII Dark Hero (BIOS 4602) | Cooler: Arctic Liquid Freezer II 360
Memory: G.SKILL Trident Z Neo 32GB (2x16GB) DDR4-3600 CL16 | GPU: NVIDIA GeForce RTX 3080 Ti FE | PSU: Corsair HX Series HX850
Drives: Samsung 980 PRO 2TB x2, WD_BLACK 10TB HD | Case: Fractal Design Meshify 2 Dark Tinted Glass