[jocko]

I'd tinker around and figure out what you have first. There are threads on here as to how to take a wag at it and guess (that are pretty accurate) that don't require removing the rear cover and counting teeth.

Since you mention you aren't opposed to a future trans upgrade, then I'd simply figure out what you have now and then save up for an o/d trans upgrade to a 2004R or 700R4. Many many folks on this forum have done that, and it's worth the effort. It is important to figure out your rear ratio beforehand - if you have a 3.07 now and add an o/d trans, you may find that your rpm is too low at mild highway speeds. 3.07 is a great highway gear, and if you're not aching to do burnouts, you might be just as well served to keep exactly what you have now and in good mechanical condition (I'd recommend doing that anyway). An o/d trans is really best suited to a tighter range between suitable rear end gearing options - my 2 cents would be between 3.42 and 4.10. Go lower numerically and you start getting lousy o/d trans performance/too low rpm in the 55-60 mph range - go higher numerically and you'll be chirping the tires at every stop light even if you don't really want to (because, in addition to the 4th gear overdrive, o/d transmissions generally have a deeper first gear than their non o/d counterparts). 3.73 is the absolute sweet spot for an o/d trans (for "most" tire diameters, different story if you have tiny tires or big mudders) and this is why GM used 3.73 in so many car/truck configs that came with o/d transmissions.

If you don't trust the counting driveshaft rotations vs tire rotations method to figure out your current ratio - if you tell us your tire diameter (measured, use a level across the top of the tire), and actual speed (on a gps, not your speedometer), and your rpm at that speed (if you have a tach) we can tell you your gear ratio. Without that info, you'll have to go old school and either count teeth or rotate the d/s and count revs under the truck.

EDIT: Btw, as Mike mentioned above, your truck would have come standard with a 3.07 ratio (assuming it was built with a 350 originally). If it was born with something optional for the rear ratio, it would be annotated on the SPID. Also note that if the 350 is not on the SPID (and your VIN has an E code rather than an S code), then it was born with a 307 engine and would have had a 3.73 rear end. If you have confidence in your truck's lineage/originality (and most of us don't, ha), then you could assume 3.07 of nothing else is on the SPID. Also, if it seems like it isn't exactly a drag racer and it isn't rev'ing real high at higher highway speeds, it's probably a 3.07.