Sockets. Get more sockets. It's all about the sockets. Then fill those sockets with +strength gems.
Also gear from Burning Crusade dungeons scales up to level 80. So a lot of that will be BiS. I believe there's a set that procs strength. Look for that. Basically look through all the gear from BC dungeons in your adventure journal in game. Wowhead is not as reliable now with all the scaling.
You'll also want the mount hyjal exalted ring (
https://www.wowhead.com/item=29301/band-of-the-eternal-champion) that procs strength and the ICC ring that procs attack power (
https://www.wowhead.com/item=52572/ashen-band-of-endless-might).
I think the band of the eternal champion works for plate DPS? Not sure.
Then whatever trinkets proc the most strength or attack power I guess.
Lastly, item level does not always tell you what is the best. There are many items that have a lower item level but for some reason give better stats than other items 10+ item levels higher. So you're going to just have to compare stats on stuff.
Lastly, there are some BC epic gems from running BC heroics that give unusually high stats such as
https://www.wowhead.com/item=30559/etched-tanzanite and
https://www.wowhead.com/item=30584/inscribed-fire-opal. Those are better than +7 strength gems IMO.
