If I recall correctly, I had a similar size issue on my 88 truck and ended up buying four different sizes (spending mucho moolah)...the bed bolts were actually T50s...but some bolts, like the ones I used on the body mounts and my seatbelts, were T55s....so you may be able to use them eventually...
Usually stores will exchange items if you have a receipt...I'd go that route if you can't use them...or maybe try them on the seatbelt bolts...otherwise, if the T55 is just a bit too big then it's probably T50...but you can also check a Haynes or other manual to see if it is specified for your year of truck...