Real nice rig.
I have a '71 GMC myself -- K/1500 Jimmy -- also a St Louis plant build.
350 is good. I didn't see a transmission listed on the SPID, so does it have a three-on-the-tree Saginaw?
Longbed Stepsides are rare. ''Longhorn'' Stepsides even rarer.
[Now the GMC purists will jump all over me, as
1] GMC didn't call them ''Stepsides.'' Fendersides or something like that is more correct.
2] GMC didn't call their 133'' chassis trucks ''Longhorns'' either. That's a Chevrolet marketing term. But if you told me you had a Longhorn Stepside GMC One Ton, I'd know exactly what you meant.]
My K/1500 Jimmy is exactly like a Chevy K/5 Blazer, but to GMC it's a K/1500 not a K/750. Go figure.
I also have '68 Chevy C/10 shortbed stepside. It was a fleet truck. Possibly Cal/Trans or Southern Pacific were the original users. [Bright Orange color is a clue.]
I bought it in 1973, when I was 23. Still got it.