Are you new to the game or something?
You have to learn to play against vehicles that have a higher BR than yours, no matter how frequent uptiers are for the specific BR you want to play.
If you cannot deal with King Tigers while using your 6.0 lineup, I suggest you either practice where to shoot them (weakpoints or hit their barrel if you cannot guarantee get a good hit otherwise) or you switch to another lineup.
M18s (Hellcats) are perfectly capable of destroying King Tigers and other 6.7/7.0 heavies. Considering that the M64 is just a M42 hull with a M18 gun, that thing also should have no problem dealing with those heavies.
Similarly, the 6.0s of other nations can deal with these heavies as well, some could have a harder time, but they all have to rely on the same strategies against 6.7/7.0 heavies. Heck 5.7s have to deal with King Tigers as well, but again, proper aim or playing strategically well lets them do so successfully.
Uptiers are basically the default, unless you are lucky and end up as one of the four players per team that gets a full downtier, you always have four or more players with higher BR vehicles than you per team.
Same BR matches only happen when there is no higher BR available.
If you have to rely on downtiers to play well, honestly, that’s a skill issue
And just for the record, British vehicles not only stand a pretty good chance against heavies at that BR, they actually tend to have an easier time disabling (emphasis on disabling, killing can take multiple shots, but Brits are used to playing support lol) those heavies, despite having only solid shot.
Spoiler
Also, the US heavies at that BR range are much better than the German ones lol