65 Valiant small block with Heddman fenderwell headers. Need suggestions on master cylinder upgrade

What I would do in your case is three fold. First, as mentioned, upgrade to a dual reservoir master cylinder. Secondly, I would get the master cylinder relocation block for the Hemi. This moves the master cylinder over towards the driver's side fender for more clearance. Third, I would employ a shield to protect the master cylinder from heat.