[dsraven]

just to clarify your situation, is the bleeder screw removed or is it still in the wheel cylinder? front or rear? if it is still installed can you clarify why you need to replace? siezed, plugged, can't get a wrench to bite? if siezed you can try some penetrating oil and heat but then you may need to rebuild the internals since the rubber brake cups and the existing fluid are not gonna like the penetrating fluid. the heat, depending on how hot it gets, may bake the rubber cups inside.
with not knowing the circumstances and/or the tools you have available I will explain how I have dealt with siezed bleeders in the past. since that is the most common bleeder screw issue I find other then the stripped corners for the wrench.
assuming the wrench corners are ok, I will ensure the wrench fits on the screw well, then remove the wrench and keep it handy, like readily accessible and also have a hammer ready and clear the area to allow a hammer swing. then I heat JUST THE SCREW with a torch till it is starting to glow red. I use an acetylene torch for this because it heats just the screw mostly and not the surrounding brake cylinder area. it is a quick heat up with the acetylene welding tip torch, not the cuttting torch, to heat just the screw part but not melt or distort the screw part. that expands the screw at a different rate than surrounding parts so it seems to break the seal of the rust/corrosion between the parts. you can put the wrench on it quickly now and apply some torque but don't try to turn it out if it loosens as this sometimes causes distortion and stripping of threads OR simply twists the head of the screw off. then remove the wrench for a few secs to allow the screw to cool off without heating up the wrench. when it cools down, so it isn't glowing and is back to iron color, put the wrench on it and then give the end of the screw a hit with the hammer. I know, this is gonna distort the end of the bleeder because it is hollow. the bleeder is toast at this point anyway. now apply some torque to the wrench. not necessarily to turn the screw but to possibly break the seal on the corrosion. try both directions a bit but understand it is already tight so it may not move at all in that direction. keep at it with the wrench and the torque applied to the loosening direction, alternating between tightening and loosening, and when it cools enough so penetrating oil won't just burn off add a little to the area. keep going with the torque. if it doesn't let up, try the heat again. you will need to remove the wrench for this so do that but ensure you are able to get it back on the screw before you add more heat. repeat until it gives up and comes out or it breaks off. if it breaks off you might just as well replace the wheel cylinder because drilling it usually means it will leak after, even with a new bleeder screw installed. a new one is gonna run you about $10-20, depending on where you buy, and they are used on chevy products from 1950 up to the mid '70's so somebody should have one in stock for a decent price. it is just the inconvenience of changing it plus the possibility that the brake line also breaks off and it becomes a can of worms replacing more and more brake lines as you go. the good in that whole scenario is that those breaking lines were probably thin walled from corrosion, between the line and the line nut on the flared ends, so they probably needed replacing anyway from a safety standpoint and would likely not pass an inspection if the shop/inspector is worth his salt.
I know you didn't really wanna hear that. sorry man.
more info and pics would help us help you. or, maybe you got it figured out already.