Adding tetraethyl lead (TEL) is the cheapest way to raise the octane of gasoline, especially gasoline that is created mostly by simply distilling crude oil. As part of reducing auto emissions, catalytic converters were required in the mid 70's and lead was banned because it poisons and deactivates the catalyst. To maximize the catalyst efficiency an oxygen sensor was also added a few years later, and the lead also poisons that. It turns out that the lead in the exhaust fumes was a major source of lead in young children, especially in inner cities, and lead poisoning causes mental development problems in young children so this was an unexpected health benefit that probably would have caused lead fuel to be banned anyway once researchers figured it all out.
To get the same octane without TEL requires a mix of further chemical processing steps beyond the initial distillation, and other additives, so gas costs more. Because carbureted engines without O2 sensors and computer controls from the 60's would run a little leaner and thus drop their hc and co emissions, in the 70's the epa decided that in the 80's gasoline would have oxygenated additives like ethanol and/or MTBE, and we would just live with the 20-30% drop in fuel economy. Yes, it is true that even with an O2 sensor and computer control the exhaust emissions do drop some, but how about that hidden 20-30% gas tax they just slid in? Oh, of course the auto makers were allowed to still use non-oxygenated gasoline when they did their epa fuel economy testing. I'm exaggerating a little because the oxygenated fuels are only required in epa non-attainment areas, but that includes most of the north east and California. When I got my car in 1990 I could get 24-25 mpg on the highway. Now, with Maryland gas (RFGII, reformulated gasoline II, third generation oxygenated low aromatic gas) I get 18-19 mpg on the highway, but when I get far enough south to get real gas I can still get 23-24 mpg. Sigh
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As for octane, the T-Bird should be just as happy on today's 93 octane as it was on premium back in '64, as far as knock and pinging. The lead also lubricates valves and valve seats, and a few engine families did have problems when unleaded came out, but I don't know if the Ford FE's were in that group or not, sorry. I have seen some little bottle of "fuel lead" for sale at auto parts stores, to lubricate the seats, but again, I don't know how well they work.
