Let me start this by saying I HATE FRONT DRUM BRAKES. And, if you look in the Archives, you might find where I've explained this before.
This is a little rusty, since its been close to 10 years since I did my conversion... and the winnie still stops like a compact car (maybe a little extra pedal effort...)
on a D20 (M-300) disc brake swap is not hard. Rear brakes are identical through the 1990's (m-300, RM-350, D-350, D-400, D-300, D3500. they all have the same 12X3 shoes, and the same single wheel cylinders.)
You want a proportioning valve from an RM 350, 1975 up. This has the right flow rate and mounts in the same place as the original 71-73 drum brake proportioning valve. (but, the valve on the later chassis was not mounted in the same location. don't expect to find it in the same place as yours in a junkyard, but using a Junkyard Prop valve is just fine, you can clean it and test it easy enough and they almost never fail. DO NOT USE the original Drum brake proportioning valve. It will not work! hydraulic flow rate in the front brakes is way different! (if you were to do that... surprise!... the only brakes you'll have would be the rear!)
Ok.. nitty gritty.... you didn't ask for this much info.
Find Spindle hardware, Rotors, caliper mounting plates (where your backing plate is for drum brakes), hardware and calipers on an RM-350 chassis Basically the whole 9 yards, but not the axle itself or spindles, they're the same! use the calipers as cores and rebuild, or buy rebuilt (they're cheap, by the way.) You will have to buy a Tap for one hole in the spindle. (I'll have to look up the size (7/8-32?), but its simple to do)
Remove everything down to the spindles on your front axle. Tap the one hole, reassemble (new seals, bearings, bearing races (not expensive).
You will need to replace the M300 Drum brake Master cylinder with an RM350 master cylinder. (Can't find one? look up a 1978 B350 Sportsman dodge Van. It's the same... as well as almost all other Dodge 1 ton trucks! they all bolt right up to that giant brake booster. (helps to have the one off the stripped RM350 chassis though, just for the warm fuzzy feeling that its the same!)
Remove and discard all brake lines, hoses, all of it. use new lines only, 1977-1990 B350/B3500 Van Front flexible brake lines are a direct fit (Pickup truck hoses are 3" too short), but there is a crimped=on bracket that will have to be removed, or just leave it on, like I did (just go for the fit on the caliper hydraulic line and the length, the brake line fitting size is going to be different
. that's why you replace the lines... and those are cheap too! It doesn't hurt anything, doesn't get in the way of steering or flexibility.) Rear Flexible brake line is still available (NAPA). Tell them it's off a Dodge 1 ton pickup, say 1978. same diameter, direct fit.there used to be a place in Boston that supported the brake parts you need, I have no idea if he's still around, but at one time, he took a member here for a really bad and unfortunate business deal on a Hydrovac rebuild and runaround.
Oh... your booster is probably fine. change the air filter, change the dry rotted vacuum lines and it should work like new... and of course work with the disc brakes too.
Everything bolts up like it was made to fit. Well, it is made to fit! wheels will bolt on with the correct Keyed and conical offset lugs, and the best thing of all? When you hit the brakes and find out there is no brake fade, pulling, or grabbing. Your Winnie just comes to a rather Quick STOP! on demand! no adjusting needed, no leaky front wheel cylinders.. nothing. Just Stopping like it should have when it was new (but never did).
my total expense (10 years ago) on the brakes, bearings, seals, everything was about $700, including the "junkyard" parts. but everything including the rear brakes was totally new. Labor was free, I did it all in about 3 days... and I'm lazy.
Hope this helps!
P.S. Here's the link to the info I posted when I first considered this swap:.http://www.classicwinnebagos.com/forum/index.php/topic,1036.msg1055.html#msg1055