I have rebuilt my front suspension, boxed my LCA's, reinforced my K frame and steering box mount, welded hefty subframe connectors in.. and the thought started rolling around my head, weld the k frame to the sub frame? Has anyone done this? It's just a thought that flashed in my brain that has no experience or research to back it up, so i thought I'd ask here, would it have any benefit like improved rigidity?

I pretty sure Hemi Fred did that in his duster.....I think I still may have pix.try a search using his name you'll probably find it.

IMHO the attachment points, and the frame and K strength at these points, is very good - which means it won't be better welded.
But it will be a service headache.
Could a lighter K be designed that required welding? Could be.

Sure, 40 years ago that is how the NASCAR boys built their cars. They moved the K frame up into the frame rails and welded it all together.

I have seen it done, along with moving attachment points in the frame rail to cheat the wheelbase forward, and other tricks. I don't believe there would be an advantage strength wise and I wouldn't paint myself into a corner by doing it myself.

If you have rubber idolators in there now
just put in alum mounts to make it stiff..
dont weld it up... just a PITA.. if you are
steel now you wont gain anything.. just make
sure the bolts are tight so it doesnt move
around

I don't think there was any benefit in mine, but I Tig welded the K to the frame longitudes and have titanium bolts holding in place of the stock ones to the frame. Very stiff I must say, but if I ever had to service it as in replacing the K, I would have to cut away the tig beads with a Dremel in order for it to detach.

What year car? If it is a 71, or older it already has special shoulder bolts in it.