For the code on your engine, it breaks down like this, first letter is the plant code, F being the Flint plant, T would be Tonawanda NY. The next 4 numbers are the Month/Day of assembly, whihc as you may notice, is only a week or so after the block was cast. Last leter is the application. ou mention the "I" that's in the code before the 9, it's meant to be a "1", and is very common to see thm stamped that way. Mine has the same "I" in it's code as well.
Something I will mention, as you've found, most decoders do not go past '57, in addition, most of them do not list anything that isn't a Passenger car or Light Truck/Suburban. The Medium/Heavy Duty Trucks and Double Duty/Step Vans tend to get left out. Much of the information can be found on the Heritage site, though.
http://www.gmheritagecenter.com/gm-heritage-archive/vehicle-information-kits.html.
