http://inlinetube.com/Prop%20Valves/PR104b.htm

I am converting 9" drum to KH disc and 10" rear drums.

I already installed the inline tube hard line set for drum brakes.

If I buy above, I'm not sure how I can plumb it in with minimal modification to hard lines.

Do I even need this, or can I just use an aftermarket proportioning valve?

If I do an aftermarket valve, can I install it between the rear distribution block and hose, or does it have to stay close to the master cylinder? (It'd be easier to bleed after the swap that way, and plumbing is way easier).

That looks like a repop of the factory "add-on" proportioning valve. I'd go with the AM adjustable unit so you can dial in as needed. Plumb it somewhere between the master & the rear hose (be sure to follow any instructions that come with the unit.)

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63 Valiant Wagon
225 - 4 bbl