The wagon has power brakes.

In the few minutes I've driven the wagon yesterday I didn't notice any shortage of booster-vacume, but my '65 Chrysler, which has the same setup, does have this problem indeed.

About when did Mopar started using dual diaphram boosters?

Whenever they started to offer Budd disc brakes as far as I know. If your car is a column shift I believe you will have to use a disc booster no later than 68 or it will interfere with the shift linkage. If it is console shift then just about any disc brake C-body booster will work. The later boosters are bigger in diameter so the shifter arm on the column will hit it.

Kevin

Pre'65 Chryslers had pushbuttons, so no 'sticks' sticking out columns and such.
1966 was the first year (Budd) discbrakes were optionally available on Chryslers.

Today I've located an alum. prop. valve I had bought once, along with a 'linelock'-installation kit from Summit. This kit has 4 or 5 different lengths of brakelines in it, and also a bunch of various sized fittings.
I would stronly suggest buying such a kit if you're doing a discbrake conversion, as it makes reconnecting the brakelines a snatch.
No flaring is needed as the brakelines come with fittings pre-installed.