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Th400 was used for nearly 30yrs. I couldn't tell you how to pinpoint the year. I'd say that's a somewhat early one.
I'd go to a sight like rockauto and look up the seal throughout the years on vehicles with a th400 ,(One ton 2wd's will work ) and see if the part number changes. Could be the same seal for 30 yrs.
In a factory setup there would be a switch on the carb or possibly on the throttle linkage under the dash. Since your truck doesn't likely have the original motor/trans you may not have one.