should I keep the k-frame

What year car are you working on?

If it's a 67, then get the later K frame because the idler arm for a 67 is a bunch of cash last I checked and different from the 68-72 idler arm. The spool mounts on the 73-76 are superior to the biscuit mounts on 67-72 K frames. You'll need new motor mount brackets with the 73-76 spool K frame.

Keep in mind, the 67-72 upper and lower ball joints are around 2x as expensive as the 73-76 and you'll be tossing them when doing the conversion to BBP suspension. Only other possibility is to use a sleeve on the upper to size it for a BBJ spindle.