Small block in a slant 6 abody 67-72 cheap and easy using truck mounts

As to truck 5.9s
The oilpan will not fit into an A-body
When running headers; you may find interference with the center engine mount which you will have to grind off.
And of course,a carb intake with all that entails
including a KD system for the trans.
You'll have to install an old-style distributor with flyweights and a Vcan.
And deal with the reverse rotating serpentine system
At the back of the 5.9, some guys have had trouble with fitting a pass-car trans and TC. I'm parroting FABO members here, so I can't say what the issues were. I guess the prudent thing would be to prefit the trans before you proceed any further.
And that's all I can think of.