Gentlemen;
In the 60s when those engines were designed/built the amount of lead (octane) in 'regular gas' was nill compared to todays fuels. :rolleyes:
It's true the seats last longer with the steel insert, but your "looking at" perhaps a 10% difference at best (maybe).
There primarly for aluminum heads is all.
The lead 'aditive' is for use in engines with the 'higher' (than stock) compression ratio(s) to avoid paying for 'premium' fuel.
"Hot Rod" (high reving) engines are always apart for valve work & they save time/labor in that application where compression, combustion tempature(s) and spring pressures are much-much higher.
For the basic (stock) Sixties or older engines don't waste your money; as the 'octane rating' of fuel then was LESS than 'unleaded' today.
"Think about it"; Will you ever put 100,000 miles on your engine rebuild, to gain the benifit??
