Has the engine been rebuilt? If so, it will have assembly lubricant already in it - modern cars often require a thinner oil for the first couple of hundred miles or so, but 20w-50 will be fine for an engine of this vintage. Once you've covered a few hundred miles, change the oil and filter. You can of course use a flushing oil - but you may find that just flushing it through with 20w-50 does just as good a job.