1971 Swinger 318, conversion to a 6cyl/special motor mounts needed

There are no "special motor mounts" for a V8 to slant 6 swap. Only if you're doing the opposite.
On a '72 K frame (applies to '67-'72), the left (driver's side) K frame mount is the same whether slant 6 or V8, so that side will bolt up with stock factory slant mount brackets of the correct year (stay away from '66 and earlier mounts, or '73 and later mounts, they are completely different designs from what you have).
On the passenger side, however; the V8 K frame mount is taller than a slant six mount (by about 2" is what comes to mind, but my mind is highly unstable :)). There is no way in he** I'd cut down a factory small block K frame mount, and if you're determined to "leave the K frame intact"; that only leaves the option of shortening the passenger side engine bracket by 2" (or whatever the actual number is).
So, break out the hacksaw and welder- that's about the only way you're going to git 'er done.

P.S.- You DO know the transmission from a 318 won't fit a slant 6, right?