i Don't know what 4/21/64 is but for almost most of the welding done on automotives needs to be done with MIG (preferably with gas). Using arch will burn through the metal (like cutting through water with a knife). Plus like stated above the arc is not as controlled as it is with MIG (i.e. you need to drag an arc to start the welding).
As others have said a mig with gas is the preferable way to go. BUT I rebuilt my 68( floors, rear quarters cowls,battery inner fender) with an arc welder with a stitch weld attachment and it passed my bodyman's critcal inspection. So it can be done but not as earily.