As for lead, an old flat-head machinist told me that he ran non-hard-seat heads on LP gas and had no problems. His point was that the LP was about as dry and non-cushioned as you could get and it did fine. He also stated that he wasn't using it for drag racing, just delivery and highway use. Thus, the seats held up just fine in his point of view.
Your mileage may vary..

 

Asking about lead substitute, particularly on a forum with a lot of people running flathead V8s, will get you all sorts of answers. Wikipedia has a good article on lead in gas. For the purposes of this discussion the part we care about is valve recession. When burning gasoline the exhaust gas heats the exhaust valve to a very high temperature. Regular steel valves and cast iron will micro-weld themselves together at these temperatures. When the valve is forced open again the weld breaks and removes a bit of valve and / or seat. The process repeats and you have the dreaded burned valve.

Ford used very good exhaust valves in the flathead V8s. Until 1951 or so Ford used very good hard valve seat inserts for both intake and exhaust valves, then eliminated the intake valve seat inserts. Somewhere in 1952-1953 Ford eliminated the exhaust valve seat inserts too. If your engine has never been rebuilt, or if it was rebuilt with the original valves reground, it will have good alloy exhaust valves. If it is a 53 engine it may now have hard exhaust seats if it was rebuilt at some time in the last 59 years.

On my cars and trucks that do not have a catalytic converter I use marvel mystery oil in the gas to help lubricate the valves. I do not like lead substitute. However the correct answer is the one that works for you.

 

There are a couple of 53 f100's with automatics around. I have one. Welcome.
Otherwise everything looks like good advice. I can maybe help with photo's of linkages etc.
they are a bit different than the manuals.

bill