Why anyone bothers to clear an old truck is beyond me... I think it looks ridiculous and serves no purpose as far as I am concerned (I know you didn't ask). The claim to "save the patina" holds no water either as it's taken 60 years to get that way, so what is there to save?
I haven't seen clear come off new paint jobs for a long time because the industry has learned their mistakes. The old paint will accept a clear coat if it's prepared properly but you might consider something else.
Take your finger with a bit of spit on it and rub it one spot and you'll see the good shiny paint and will have the oxidised paint on your finger. Take the old oxidized paint off and you will be surprised how much shine you can get. Safest method is to do it by hand with a fairly coarse compound and a rag. Follow up with a wax, and you'll have shiny patina.
