The best way is a core plug block heater if you can find a suitable low core plug that works.
something like this:
https://www.jegs.com/p/JEGS/JEGS-Freeze-Plug-Core-Plug-Block-Heaters/752718/10002/-1
I don't know which core plug would work for you, so you either need to wait for somebody who does, find it somewhere else, or take your chances that there's clearance for the heating element. I've never used the Jegs heaters, and never seen block heaters with 1,500 watts either, usually you only get 400-600W.
If you use a tank type circulating heater, the heater has to be below the connection with the top hose sloping up to the connection to the engine. At least the cold coolant from the engine has to come from low on the engine, the heated coolant can go to the head or just an inch away from the cold hook up on the block, even the same port if you have a 1" or bigger port. The heated coolant doesn't matter, it will circulate up on it's own. The hoses can't trap air either, they have to slope up to the engine to burp.
Maybe this will help:
https://www.heavyequipmentforums.co...e-in-the-heck-does-the-block-heater-go.47898/
If I understood that right, use the heater hoses to tie in a tank heater, that might be what you have marked in the pic.