Converting it on my 66. How hard is it and what do I need?

if manual then a M/C with the recess in the rear of the piston "thimble" and a snip of vac hose the length of the recess at the end of the pushrod & of a diameter that you can just slip over the rounded nub. If power then a M/C with or without the recess in the piston thimble. Connect the port next to the firewall to the current line (it's larger dia so need a brass adapter (Edelman)) and connect the other M/C port thats closer to the radiator with a new 3/16" brake line down to the splitter block and unscrew the line going to the rear brakes & connect the two together with a brass double female inverted flare 3/16" fitting from the "Edelman" fittings cabinet at your parts house. Could go to 7/8" rear wheel cylinders or plumb in an adjustable prop valve in the rear line. Ideally you'd want the correct M/C bore size depending on if power or manual and if drum or disc & people have reported leverage problems using the wrong bore M/C but I used a truck M/C (1&1/8" bore iirc) M/C on a 65 dart swap with 10.87" front BBP discs/10" drums and no issues & no prop valve either but if I stand on it (which I rarely need to do) it will lock the rears first. If (re)using the same pushrod, the 4bolt/2bolt adapter spacer because of its' thickness will move the M/C forward and lower the pedal some plus the alum M/C likely will have the piston recessed in further, further lowering the pedal so mock it up & check & there are several length of manual brake pushrods in OE production or just get the MP adjustable (Mancini etc) pushrod or drill/tap to make your own from 2 of em if you're handy with taps/dies. EDIT on page 2 CNXT posted a M/C bore chart which may help guide you

Everey one Ive done we used 70 C body masters.Either disc drum or drum drum.If you are going disc you need a porportioning valve.I pulled some from later Mopars.I also pulled the dual blocks from Mopars at the Junkyards when we do drum drum.Rocky

Sorry, I forgot, I am staying drum brakes.