Looks like a really clean rig to start with. Rockers are really common to rust out, I'm in the process of repairing mine on my truck, so far not so bad.
As for the VIN:
https://www.usa1industries.com/1973-...der-chart.html
Chevrolet
2 wheel drive
305 V8
10 (half ton)
Did the 9 and F get out of order? If I swap them
9 = 1979
F = assembled in Flint, MI
The 6 in the body type must mean suburban.
"Heavy Halfs" did exist but given yours came from the factory with a 305 and as a 2WD, which yours obviously is not anymore, i'd guess it might be frame swapped with a K20 (or maybe just axle swapped).
From what I can tell, the 203 shifter should look like this
205 looks like this:
I'd be somewhat surprised if your rear axle was actually an 8-lug 10 bolt. Usually the 3/4 ton axles were 14 bolt semi or full float rears. Dana 44 or 10 bolt in front is common though.
Hard to diagnose the transmission issue without experiencing it. The SM465 is only 3 speed but I can drive 65-70 without issue in 3rd gear.
I think the 1/2 ton springs are usually 2-3 leaf in rear from what I've seen. A 3/4 ton was commonly 3 spring in front and anywhere from 6-9 leafs in rear.
How does the engine run?
I would personally start with replacing every fluid, differentials, transmission, transfer case and engine. Inspect oil for metal.
From my experience, NV4500s are fairly difficult to come buy these days and if you do find one, expect to pay $1000-2000 for it.