I usually have a few universal mounting kits laying around for this purpose.
Are you using ATMOS speakers? If not, just use another setting in combination with a digital optical cable, forgoing the ARC connection.
I just upgraded to a 65 inch samsung q70r qled tv from a sony xe90 55 inch led. Everything seems fine so far apart from a few things. I don't have any sound coming from hdmi arc. This is for dolby atmos hooked up to the Onkyo HT-S5805 5.1 home cinema system. It worked fine on the sony but there's no sound coming from samsung. It's all connected and it's showing up in the samsung sound settings but just wont produce sound. I'm also having problems playing video via usb. For some reason the picture on my usb videos look strange like very high frame rate and general bad picture quality and ghosting. Again, this worked fine with my sony xe90. Is there a better video play I can download like vlc? Last question is for the wall mount screws. For some reason samsung didn't include them with the tv. I know it's m8 screws but I can't seem to find out what length they are for my screen. Any help would be much appreciated, thanks.