If your valves have not been replaced with hardened versions, then lead substitute will provide lubrication and cooling to prevent burning valves.
The pump fuel maybe ok depending on compression ratio of engine. The higher the compression the higher octain you need.
If compression is below 9:1 regular is probably ok, over 10:1 octain buster to premium.
Others my have better breakdown. Timing can also be retarted to burn lower octain but power suffers.
A friend of mine that does head work says the valves are hard enough from the factory. I drove my previous Cougar for 5 years on just regular or premium unleaded, never added anything and the valves were fine when I rebuilt the engine. Octane is important if the timing is advanced some. I usually adjust mine to burn 87 octane unless I need maximum performance.
ssmorby , its not really the vlaves it hurts its usally the seats. How were yours? I really think it takes a steady diet of no lead for quite a long time for it to really hurt them but I think at rebuild its a good idea to go ahead and put in the hardened seats anyway. mm
here is where having a ford pays off. ford used a higher nickle content in its forgings and their heads are the hardest made followed by gm and if you have a chrysler your SOL because those are some soft seats. So run it on gas that doesnt make it ping then when its time for a rebuild put the seats in but it takes a really long time to wear as mark said. An old trick also is through half a cup of tranny fluid in with a full tank of gas it helps lube the valves and cost a group less than lead substitute.
Erik, don't forget that octane ratings are not the same as in the good old days when our cougars were new. Regular was around 94 octane and premium was around 99.8 !! I put the highest octane in my 67 (93 in my area).
When my Cleveland was rebuilt, there was no room to install hardened seats due to the size of the valves. I dropped the compression to 9:1, and add lead at every fill up. I ran 92 octane for awhile, but decided it really wasn't necessary. Haven't noticed any difference in performance, or operation. I suspect in this configuration the original 300 HP is probably down about 50 HP. Anyone have a better estimate?