It seems very hard to make a 320K resistor like this.
Really? Two 160k
1/
8W resistors end to end in series isn't difficult.
1. Twist the leads between the two closely together over a short distance, solder them together and trim the excess lead length, essentially making one
1/
4W resistor.
2. Insert into PCB and pretend it is one resistor.
You can simply "tent" the joined resistors up a bit if you've made the part too long to fit directly between existing holes. Job done.
Unless you have very limited space above the PCB this is not a problem. You can even join the two side-by-side, one going up and one going down to the PCB instead of soldering them end to end. Resistors are often mounted vertically to save space, there is nothing inherently wrong with standing up a resistor, it's not like they know what orientation they're in.