What motor would a regular chevy s10 v6 automatic come with?

A 91 S-10 probably had a 4.3 V6. You need to look under the hood or ask the parts guy at the counter to look at it.

You could also find a vin# decoder online, and type in your 17 digit vin# that is located in the bottom right corner (drivers side) of the windshield, if you are looking at the vehicle from the outside in.

Or read the emissions sticker under the hood.