Since you still have the original 307, I'm gonna go out on a limb and guess your rear gears have never been changed, but you never know till you crack it open and count. Your truck would have come from the factory with a 3.73 rear end, which shouldn't feel like 5000 rpm on the highway unless you're doing 90. So maybe it has been changed out.
Engine has nothing at all to do with highway rpm, only trans final drive ratio, rear gear ratio, and tire diameter - all of which have been mentioned.
All factory transmissions that were available in these trucks, regardless whether automatic or manual, have a 1:1 final drive ratio.
Since you just bought new tires (and they look great, btw) I'm assuming you don't want to change them to something taller. Tire size, unless you go with LARGE diameter tires, as in a lifted 4x4, aren't going to make a "lot" of difference in your highway rpm - they'll only reduce it slightly if you go a little taller (and your speedo would then be reading slower than you were actually going, ticket magnet
)
That leaves the trans and the rear. 3.73 is a good all around gear - especially when coupled to an overdrive transmission. So, personally, here's what I'd do:
1) determine actual rear end ratio - crack it open and count, that's the only way to be sure.
2) if you confirm it's a 3.73, and you plan to keep the 307, and you aren't a burnout fanatic, I'd swap in a T5 trans. Search on here for the T5 swap threads, there are several. It will drop your highway rpm significantly and also increase your driveability.
I assume you don't use the granny 1st much, and generally start off in 2nd? If so, you'll like the increase in apparent acceleration in 1st compared to granny 2nd starts. If you do use 1st gear on your current trans, well, you won't miss it and you'll like the T5 better. It is a fairly deep 1st gear also, and coupled with a 3.73, you will be shifting to 2nd by the time you hit mid-intersection.
IF you determine that someone has swapped in a 4.10 gear, I'd still do the same thing.
IF you determine that someone has swapped in a 4.56 gear, well, I'd probably swap the rear end in this case and leave the trans alone (or do both if you can afford it). I'd swap to a 3.73 or lower numerical ratio (higher gearing) if your goal is highway driving. If you are going to swap rear and trans for an o/d trans, I'd always opt for a 3.73 - it is truly the sweet spot for any o/d trans - best of both worlds.
Based on your described goal (lower highway rpm and you could do without the granny) and the likelihood that you still have the original 3.73 in it, a manual overdrive trans is your best bet to achieve your goals. Leave the engine and rear alone. An engine change will not affect highway rpm no matter what.
Recommend compare your speedometer reading against a GPS speed indication. If they are close - that's another indication you have 3.73 gears. Back in the day, if someone swapped a rear gear, they USUALLY didn't go to the trouble of also swapping out the speedo gearing so that the speedo would read correctly. If your speedo is notably different than the GPS reading, then I'd suspect a rear gear swap - in which case you'd definitely want to crack it open and count teeth. Might want to do this anyway, change the fluids and get free peace of mind on what your actual gearing situation is before you plunk down $$ on a transmission.
One more note - if you DO intend to swap the engine out at some point to a higher hp/torque engine, the T5 might not be the best bet, unless you know you will drive it sanely, no hairy burnouts, etc. I had a 283/T5/3.73 combo and it was a lot of fun to drive.