On a G reg, it's almost defo a pinto, the CVH didn't come in 2L size so it's not that.
How wide is the rocker cover on your sierra, is it flat about 10" to a foot wide, if so it's DOHC, the pinto cover is much narrower about 6-7" and rounded/ish over the top.
Did you tell them it was a AUTOMATIC ?, as the starters are normally specific to auto or manual, certainly on the pinto a manual starter will not work on a auto, the nose of the starter is a different depth, due to the teeth on the flex plate of the auto being a lot closer to the engine than the flywheel teeth of the manual box.
tabetha