magnum 360 into a 74 duster that had a slant 6

Hi Eric...and welcome. Great to have you here.
Lots of great info on magnum swap.
As far as headers go Doug’s, TTI and Schumachers are prob the best bets on fitment. How much are you looking to spend becomes the real question. Lots of threads here on all of these. Again some questions relative to this would be power or manual steering? Stock torsion bar suspension? Already have the 727 so floor or column shift? Are you looking for long tube or something else?
Would you plan on using the magnum or LA water pump and pully system? All of this is relavent.
I’ve personally never heard of an HEI distributor fitting without issues on a small block but an HEI conversion works very easily with the stand alone 4 pin and mopar electronic distributor. Again lots of threads here on this.
This site is like a cornucopia of info so dig in.
Love to see some pics.