Can you post a picture of the SPID (sheet pasted inside the glovebox door)?
What engine was it born with? Does your VIN start with TK"S" or "E" or "M"?
All these things can help determine what your truck was born with and what it has now. Keep in mind that your speedo may be way off depending on tire size (use a GPS to determine your rpm at an actual GPS speed).
If your truck was born a V8, as Tyler pointed out, your stock ratio would have been 4.10 (in other words, if you still have your SPID, and it has an "E" in the VIN (& 350 appears on your SPID), but NO rear ratio appears on your SPID - your truck was born with 4.10s).
But since who knows WHAT someone may have swapped in over the last 49 years - with GPS speed, eng rpm at that speed, and tire diameter, we can tell you precisely what you have now.
Look fwd to some pics! We like our K20s around here
