I am sure that with enough digging I could find this information but will ask here first.
1980 F350 DRW, 400, C6, 4.10 Dana 70, 235/85-16 31.6" overall diameter tires. I did not drive this truck much before I had to remove the engine for rear main seal, while the engine was out I bought new tires. I dont know what it was like with the stock sized tires, so nothing to compare it to.
My guess is that 60MPH will be giving the engine quite a bit of RPM. ???

 

Smaller tires will increase engine RPM but the tires you list don't seem that much different than what it would have had originally.
I would guess that engine speed increase would be negligible in what you have described.

 

Engine RPM Calculator | Spicer Parts

Check out the link to see roughly what rpm the engine will turn, I say roughly because the fact that the C6 doesn't have a locking converter means your actual rpms will be 100-200rpm higher.

 

As for the gear ratio there may be a tag under a cover bolt on the axle.
Also if you still have the cert. sticker on the left front door post, post up the info as we can then find what trans & gear ratio the truck came with from the factory.
As a WAG I would say 4.10 gears in a big truck like that meant to pull heavy loads.

Now note this was not all that bad back in the day of 55 MPH speed limits and non-over drive transmissions.
Today everyone is use to over drive transmissions and the low RPM the motors turn when they in a old car / truck with out over drive the motor is screaming going down the road.

BTW I have a 76 E350 van, 460 / C6and I am sure 4.10 gears, I used to pull a 2 car open deck trailer all over New England and also used it to pull a 1 car to Cali and back from CT in the early 80's.
Not once did I ever think the motor was screaming. Thirsty yes but not screaming

If you want to drop the RPM some Gear Vendor makes a bolt on over drive you could get.
Dave ----

 

Pics as requested

 

Looks somewhere between 2800-3000 RPM at 60. I never had the chance to drive the truck on the highway before rear main seal leak was a flow vs a drip.