Have the heads done with hardened valve seats.
the 69 used leaded fuel, you can get the heads redone and have hardened valve seats installed and you can use unleaded with no problem.
None is needed. The car came from VW with hardened valve seats, and by now this specimen has had the heads changed--and all aftermarket heads have unleaded valve seats.
Yes, you have 2 options. First, mix in some lead substitute with any tank full of unleaded, available at most parts stores. Seond, you have to remove the cylinder head(s) and have a machine shop install hardened valve seats. Then, you can use all of the unleaded you want. The reason for this is that in the pre-unleaded days, the lead in gas served as a lubrication for the valve seats. When the wonderful EPA banned leaded gas, the lead-lubricant was gone. The car makers compensated for this by hardening the valve seats, making lead uneccasary. However, using unleaded in cars bult before about 1974 or so will result in fried valves/ valve job. I'm told that the notoriously bad valve life in old Volkswagon beetles was a direct result of unleaded gas being run in these engines.
If it's got the original valve seats in it, I'd recommend using Lead Substitute in your gas. If you've got stellite valve seats, which by now you almost certainly do, unleaded is fine.
The classic engines were built when leaded gas was standard. If you don't burn leaded gas or add a lead substitue to you gas in one of these classic cars, it damages the valve seats. If you were rebuilding the heads of a classic engine these days, most people would ask to get hardened valve seats installed so you can burn unleaded gas without a problem. But since we have been on unleaded gas for about 20+ years now, I would assume if the car is currently driving on a regular basis, it probably has had those valve seats already installed.
Many people will modify the engine by adding hardened valve seats to compensate for the lack of lead in the gas. Lead additives are also sold to add to gasoline for use in classic cars.
Have the engine heads rebuilt using hardened valve seats.
You can use unleaded in a low compression engine such as the in-line 6 and the 260. But use a high octane unleaded instead of regular unleaded. If however you have the 289, I am not sure on this engine. The 289 HO will require the use of a VSRP additive at each fill up. If your 289 has hardened valve seats and does not require a VSRP additive, you can simply switch to Shell V-Power (98 octane) or Shell Unleaded (95 octane) straight away. If you find that your Mustang tends to run hot then you will need to readjust the fuel/air mixture on the carburetor for the modern unleaded fuel.You can use unleaded in a low compression engine such as the in-line 6 and the 260. But use a high octane unleaded instead of regular unleaded. If however you have the 289, I am not sure on this engine. The 289 HO will require the use of a VSRP additive at each fill up. If your 289 has hardened valve seats and does not require a VSRP additive, you can simply switch to Shell V-Power (98 octane) or Shell Unleaded (95 octane) straight away. If you find that your Mustang tends to run hot then you will need to readjust the fuel/air mixture on the carburetor for the modern unleaded fuel.
The only things unleaded gas could screw up in that engine are the valve seats. Volkswagen started putting Stellite (a very, very hard metal) seats in their engines in the 1960s, so you're fine.
If it's a real old engine ( 1979 or older ) they do because the valve seats are soft not harden,and todays fuel will take the valve seats out of the heads. If you are going to use old heads with todays gas they will have to have hardened seats put in the heads.
The valve seats in 'unleaded' engines have to be harder then the older 'leaded' engines. Designing, tooling up, manufacturing, and installing harder valve seats for every type of engine they build cost the automakers lots of $$$$$$.$$