I can tell you this about unleaded in old Mopar Flatheads.
First, it's almost impossible to kill them.
My brother owned a stock 39 Plymouth a few years back and for 2 yrs it was his daily driver.
He averaged at least 150-200miles a day in it.
Drove it at highway speeds 65-70mph to & from work(100 miles round trip)
The ONLY problem he ever had with it was in the summer when the temps were in the high 90's.
The carb would boil over when he would turn the mtr off because of how hot it was(85-90*plus).
A thick carb spacer seemed to fix the problem.....joe