[jocko]

assuming your speedo is accurate (I'd confirm with a GPS before doing anything else), ga-stepside is spot on, it's a 4.56.

Red dashed line below is your config - even if you have a different muncie, and not a granny 4 speed, they all have 1:1 4th gear, so highway rpm is as shown. Chart below shows what you'd be looking at for RPM with multiple different overdrive transmissions - or if you changed to a 3.73. Or both. A couple notes -
- I wouldn't go with a T5 unless you were just cruising with it. They are great transmissions, I've installed one, but they aren't built for burnouts in a heavy truck or for trailering or heavy loads.
- If you want to stick with a manual trans, google on here for some NV4500 (or even 3500 if light duty use only) swaps.
- If you want to take the leap to an O/D automatic, most go 700R4 or 2004R. I only showed the 700R4, they are similar enough that you get the idea. Goods/others to both transmissions - but the swap is very common.
- Rear gear swap "only" will help highway rpm - but you will definitely feel the difference in the seat of your pants - as in, the thrill is gone. You will lose acceleration in 1st gear. O/D transmissions are great for keeping your bottom end acceleration snappy (most will increase it) and reducing highway rpm. In your case, however, a 4.56 is SO deep that an O/D trans will only do so much for you - AND, probably more importantly, it will almost render 1st gear useless (or even 2nd gear if you have a granny 4 speed) - you'll need to shift at around 5 mph into 2nd (or third in a granny 4 speed!) if you retain the 4.56 rear. Lots of things to consider. The sweet spot is, of course, the most expensive - i.e. O/D trans with 3.73 rear. Best of all worlds and why GM used this combo so often in later years.

So, a lot depends on what you want to do with the truck. Hope this helps! (oh, and by the way, coils were standard for your truck, leafs optional)