For these old trucks, the drum brakes up front are totally adequate. Although not as efficient and subject to fade under heavy/frequent braking, it isn't likely that you'll exceed your braking capacity under normal or light hobby use.
I don't know how complicated the disc brake conversion is...hopefully someone can shed light on that. You may get improved braking but there may be second order side effects that degrade your steering, suspension or wheel/tire fit. You'll also need to replace the master cylinder with a dual-reservoir proportional system...and I don't know what kind of room exists under your truck to fit that. You might need to modify the crossmember.
You can still get all the parts for the stock brakes from Joblot in NY and Mac's has some too. I don't know if you can get new shoes but you can have yours relined and recurved (definitely recommended). You can get new wheel cylinders and/or rebuild kits as well as new master cylinders. Worst case you can even have your bad cylinders resleeved and rebuilt. Springs and hardware are available as well.
Hope this helps...
