.7mm compressed is the thinnest, B30E gasket, so the shortest O-Rings on top.

You sound like you know what you are doing, I have to ask where was the leak coming from on the old pump? If there was no leak from the weaphole, then there is nothing wrong with the pump. The heater pipe on the side, is it not too rusty for it's ring? Is there a chance the leak is from the headgasket along the side under the manifold which is running around to the front? I don't know what angle the motor is mounted having not been lucky enough to see one in the flesh.

The O rings need nothing, luckily Hylomar does no harm. A tiny amount of hylomar is all the gasket needs and NEVER use silcone garbage on a donk.

Also, does it leak without pressure? I've never run a pressure cap on a B18/B20/B30 powered car, but you would need one for 4wd.