I found some great info on another site!!
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VCCA CHEVY FORUM I just went thru an extensive ordeal with this issue. I had head work done on a 1952 235, and a 1954 235 Chevy truck motors.
Here is my conclusion based on 3 machine shops and advice from 3 mechanics/machinist with over 100 years total experience.
The stock Chevy exhaust valves, as Tony said, have the lower valveface portion consisting of a stainless type alloy as they will NOT pull a magnet; the upper stem will. These valves do get burned, but that was common with leaded gas use.
The problem with running unleaded gas IS NOT that it is just "unleaded" but it often contains seasonal, emmission, or methanol additives. Worse yet, cheap "discount" gas goes rotten-super-bad over time! These additives really gunk up on an old low rpm, seldom run valves. Thus the problem of burning out valves isn't eliminated with the stock stainless valves.
The real problem is the erratic "detonation" of some of the unleaded gas (ie cheap gas)used in an old engine that "super heats" the valve seats. The newer model truck seats are often factory "flame hardened" to resist this heating and subsequent "cracking" but it still happens a lot in newer trucks and cars. What happens is that the valve seats get real hot from the unleaded erratic detonation (some say because of lower octane), anyway the red hot seats get pounded by the exhaust valves and become either cracked or very malleable. Both bad news.
The old leaded gas both raised the octane AND provided the valves seats with a "lube" or "cushion" of depositions. New unleaded does not.
Cracked seats really show up in newer Marine 454 Chevy engines. This is because somebody uses "discount" gas, runs the engine full throttle most of the day, then pulls a 3 skier load. WHAP! The valve seats are pounded into mush.
Our old stovebolts are mostly low compression engines that can burn relatively crappy unleaded gas of low octane without showing head cracks under magnafluxing. We don't often pull a heavy load in these antique trucks! BUT if our engines have been bored to replace worn pistons; or milled to reshape a minor head warp then the compression ratio goes UP and so does the risk of cracking seats! {My old stock 1952 bored to .060 with the stock head is now a 244 CU/inch with close to 9.0:1 compression! Just cause of the bigger pistons!}
The hardened inserts will expand at the same rate as the cast iron head , which is great, and they will provide a hard "anvil" like surface for a pounding exhaust valve to hit, even with overhot crappy fuel burning. This is VERY important in an old truck running at highway speeds with the stock/high rear axel ratios. Running a mid 1950's or older Chevy 6 cylinder beyond 3400rpm for extended periods really stresses the whole engine and especially the old valve train!
In conclusion, I chose to have the hardened exhaust valve inserts put in at a extra cost of about $75-90. I want these valve jobs to last as long as possible and I just don't know the direction that future gasoline mixtures at the pump will take. I want the old stovebolts to run well into the 21st century and hardened seats were cheap insurance and an inexpensive engine update that may help the old truck pull a load; or better yet just not give any future valve/head problems.
I will also burn "name brand", good quality, higher octane unleaded gas. Just in case!
PS..Lead substitutes work A-ok in stock engines that don't YET need a valve/head job: BUT are a bit "dirty" in deposits on the valves,pistons, and plugs. Who knows how long the EPA will allow them? Use em' till ya need a valve job , then inserts.
