Cashedup.. 300-400m is the normal spot out the back of Mayor Is, 90% of sounders wont pick up fish out there in that depth But will pick up the bottom.
Starting to get a little late in season for them( best time is April-May) but have caught them all year round but mainly small fish.
Yes, they do "stack up" from the bottom, mormally 50m or so but of course you will need a good sounder and ability to decipher it. Drifting over pins and or putting down droppers is a good way to find fish, especially around slack water and preferably early in the morning.
Good setup is a TLD 25 or 30 loaded with 80lb braid and 2 hook dropper rig with 2 circle hooks, 12-16/0 would be good and 32oz sinkers are sweet.Heavy jigs(500g) or diamond eye jigs in 4-500g work well also
Good luck as they are one of the best eating fish in the ocean

Anyone find more marking or were to go

If you don't have a sounder that can find fish at 3-400m it will be a lottery. Even if you had a GPS mark - that will be a starting point. After that you are fishing blind.