II brakes are not all that great but with good fluid, maintainence and some TLC they can work well for day to day driving. That said I am in the process of changing mine so that I can open up rim possibilities with the more common 5-lug configuration.
lots of options for the front... you can go with the Granada rotors w/ GM calipers and lots of variants on this such as aluminum calipers, etc. These kits only cost a couple hundred dollars and are good enough for most any driving you'll do on the street... Wilwood makes a 5 lug kit that is pretty awesome if you want some serious stoppage however it is expensive.
there are few options for the rear that don't require converting to 5-lug or getting really inventive with parts from other cars. Very little braking is actually done by the rear brakes so unless you are looking for 5-lugs I wouldn't go through the trouble.
I used to autocross mine in C Prepared, with slicks, solid suspension and stock brakes. They worked fine back in 1983, but under today's standards are not the "trick" item.
Point is if your driving it on the street they should be fine. Get them re-done by a quality mechanic, with quality parts and you should be fine for everyday driving.
I agree with Dave, they were ahead of their time when the cars were built and in every day 'normal' driving do just fine , granted everything is adjusted (referring to the rear drums) and working properly.
Of course in comparison to todays driving habits, if you're one to ram-rod it up to the stoplight and then pile-drive your foot into the binders.... you're asking for failure. If that's the case, then, yes it may be wise to upgrade to a Granada rotor setup or even trickier setup byt Wilwood.
Keep in mind, poorly adjusted rear drums will reduce your braking efforts, overall, buy up to 40%! Also leads to excessive wear or premature failure of the front brakes. Most people don't realize how important the rear brakes really are and 9 times out of 10 the drum brakes are not even barely working or damages/worn out.
Last year I removed and took apart my Stang's brake proportioning valve and cleaned out the varnish, checked the O-rings(they were perfect) and reassembled it. My front brakes worked immensely better afterward. The valve is pure brass inside and out, so it will last forever, but it does get pretty dirty in there after 28+ years of service.