An original big block K frame requires a special drivers side motor mount which is hard to find, costs big bucks and it also requires the block to have the right mounting boss cast into it, some blocks don't have this.
The old Direct Connection style K frame uses stock style mid 70's spool type motor mounts which are easier to find and will bolt to any block but space gets a little tight where the motor mount and the steering box want to live in close proximity with each other.
Header clearance generally sucks on a big block A body regardless of which "stock" style K frame you choose, the motor mounts aren't in the way of the pipes, it's everything else.
Hedman B body headers do fit a low deck engine in an A body, I have them in my car w/Direct Connection style K frame. The passenger side needs one little dent down by the torsion bar but the driver side needs quite a bit of mutilation to clear the steering box and column, but for how cheap they are compared to other brands I feel its a fair trade off.