An A-body k member weighs about 30# depending which year or engine mounts are on it.
It is a difficult part to produce lighter and remain as strong. When you really look at It is a very weight efficient designed part.
Problem can be it may be not be configured how you like it.
It has a small amount of room to be lightened but not alot. Elephant ears for example when the
engine mounts are cut off may save a couple pounds.
Most k-frames are replaced as a way to change out the steering and torsion bars to a more modern design or for easier header fitment.
It is a very costly swap when compared to even the best fitting or expensive headers normally used with the stock frame. Cheaper to buy the headers.
Some like the rack steering that is normally incorporated with a tubular "K".
Some don't find any flaws with the stock manual steering box which is also very light and
difficult to improve on from a weight perspective.
You can choose about any method to get your car lighter known and it will be cheaper per pound lost than a k-frame or suspension swap.
Better spent dollars on a lighter front brakes, fiberglass body parts and aluminum engine parts than a tube frame for my lightweight needs.
I sure some will disagree with me on this. I'm OK with that. It is your money after all.
If you are serious about getting a car light you wouldn't even be considering a k-member at all but a full tube chassis. Thats when it gets real. Not the pretend bolt on stuff for street cars.