It was almost certainly cheaper to get the small block and if that's all you need why spend more to burn more gas? The 1988 GM Medium Duty Truck brochure lists the 5.7L (350CID) V8 and SM-465 as standard equipment. I have no idea what the sales numbers were but it's not far fetched for a 1988 truck to have been configured with a small block and a 4 speed from the factory.
https://xr793.com/wp-content/uploads/2020/02/1988-GMC-Medium...
Nope, factory. I have the brochure to that truck and clearly remember the gasoline engine options: 350, 366 and 427 (tall deck big blocks) The 350 + 4spd was billed as a low cost drive line option for local work trucks that did not travel long distances. It had a rater tall rear end (tall is slang for deep reduction ratio) something like a 5:1 or better. On a flat road with your foot to the floor it could barley approach 50 MPH.
Sounds like someone swapped a 70s-80s engine from a lighter application in.
I don't think that truck would've had manual choke from the factory. Lots of stuff could've happened over the years.
The amount of air your engine breathes is monumental compared to what the smog pump moves. The math of dilution just doesn't work. What does work is pissing a light stream of oxygen (remember, not much of that coming out of the engine, especially on warm up while it runs rich) to help the catalyst burn those hydrocarbons off of itself a wee bit faster.
I'm not sure if an 80s gas MDT would've had cats from the factory.