In 83 GM changed their master cylinders from small pilot to large pilot so a pre 83 master will fit on a 86+ car but not the other way around. I prefer a later 80's
B body caprice master over the 79-81 WS6 Trans am because it's aluminum body with the better plastic rubber seal lid vs. the cast iron T/A one (looks more factory) and the pilot bore is the correct size where it mates into the booster. Other options are late 90's early 2000's RHD postal S10 master's and mid 90's
Impala SS but you need to reflare the brake lines at the master. The late 80's
B body caprice wagon master jus needs the front and rear lines flipped at the master, its a direct bolt on part and fixed my issue. Willwood and other aftermarket companies also make ones that work but I prefer to put OEM type parts in my stuff because you can get replacements at parts stores if they crap out when you are away from home.
I noticed poor brake pedal feel when I put rear blazer disks on my stock master 86 car. I tried swapping the proportioning valve firs and it didn't do anything and the valve started leaking very quickly. If you have poor pedal feel (it's squishy and seems to have too much pedal feel) you need a larger bore master. Rear disks take more fluid volume so you need a large bore master.
For what its worth I suggest keeping the stock proportioning valve until you get the pedal feel fixed. The valve will make the rear brakes apply at a different force relative to the front brakes so the time to swap that is if you are going 55mph and stomp the brakes and the rear brakes don't lock up when the front ones do or vice versa. But it won't do anything for pedal feel. I'm not a huge fan of aftermarket disk/disk prop valves anyways, most people that put disk conversions on their cars are running large wheels and tires or pair it with larger front brakes, along with changing the weight bias due to engine swaps. If you don't like how the front/rear bias is with the stock prop valve I'd go to the adjustable willwood piece (summit has a store brand that's 1/3 cheaper also just without the wilwood logo but the same piece).