I think any engine swap puts you into street modified. You'd probably normally be in F street prepared or even in one of the stock classes with a first gen where you'd be more competitive. Unfortunately SM will have some nasty cars in it - you likely won't be up there in the standings but it will still be fun.
Not a huge auto crosser, but I do play when ups hold their event. IMO, a v8 is too heavy in a small pickup like that. For anything handling oriented, ou want as close to a 50/50 weight distribution as possible.
I personally would swap to a 4.3, if I was to stay gm. If you have the ability, or can afford to pay to have it done, I would swap to a sr20det or something lightweight but powerful, to stay as competitive as possible.
Swapping to a V8 with aluminum heads, water pump, intake(especially EFI) only ads 70 lbs. Not what I'd consider a lot of weight. Unless you move the entire engine/trans 12-14" back will any engine/trans combination ever approach a 50/50 weight distribution in an S-10 truck. Go have fun with what you have. Change out suspension components; bigger sway bar stiffer springs, lowered, quick ratio steering, wider wheels tires with racing compounds. The likelyhood of any S-10, stock or modified setting "Top time of the day" is pretty remote. You want to be really competitive, buy a fast sedan, pony car or a sports car. Again, go have fun with what you have and make those kind of decisions after you are convinced that Auto Crossing is your cup of tea. Just sayin'.