My intentions with this conversion was to use the stock spindle, without any modifications, so there are also no changes in steering-geometry, alignment issues or balljoint swapping.

I took the wagon out for a drive up and down the street today just to find out how the braking-system works with the original single-pot mastercylinder still present and also stock distribution-block.
Pedalpressure is pretty much the same as before and braking feels safe and secure again.

But, as expected, the rear wheels will want to lock up, but only during firm braking. Driving and braking like a granny stops the wagon nicely though.
Then again, a stationwagon has more weight over the rear-axle, so a regular passenger-car will have the rears lock up earlier I'm sure.

The current drumbrake mastercylinder mostlikely has a residual valve incorporated inside, so it will always pressurize the brake-system with something like 10 lbs of pressure. This is good for drumbrakes, but not for discbrakes, as this will cause them to run hot very fast.

So I will be installing a used alum. mastercylinder from a late '80s Voyager since I have one laying around. Perhaps the diameter is too large and will cause a firmer pedal, but that's something I'll have to find out with trial and error.
I'll also be splicing up the current brakelines and replacing the stock distribution block with a proportioning valve.