In my 258 that has a the rocker bosses machined for roller rockers{which do not fit from a 401} I used rocker pivots from an Oldsmobile 350 V8, and pushrods from a 292 Chevy six to compensate for the machining after I had to go back to stock stamped rockers...... But, that was just me....
The later 258 Motorcraft distributor will fit in place of your Presolite ignition used in 77. Presolite parts may be getting rare, but the Motorcraft parts are everywhere. Something to consider unless you want your 77 to stay correct. It is possible to use a HEI Chevy 250 inch 6 cylinder distributor in the 258 if the drive gear is changed. I have never tried this, but have heard it works, if you prefer a HEI over the Motorcraft.
Thermostats from a Chevy small block or Olds V8 will fit the AMC 6. Stant makes a 205 degree thermostat for these engines, just in case you live in a cold climate.
A Ford Motorcraft 302 carb will fit the AMC 258 2bbl intake if the Mr. Gasket #1937 adapter is used.