The 4.3L has been a pretty serviceable engine in Lemons, which is kind of par for the course with logical thought processes in Lemons. They're generally understressed, can take a beating, and you can run one with a rudimentary carb setup. That has some appeal for Lemons; see the ORCA Racing CTS guys in Texas who threw out the Opel V6 and instead of putting a 4.8/5.3/6.0, they put in a 4.3L with a two-barrel. Looks hilarious, can be fixed with a big rock and swear words, and makes enough power to get that boat to mid-pack lap times and Top 20 finishes. In reality, I think they're old-school guys who are just more familiar with carbs, so the 4.3L is supremely easy to find for replacements.
The S10 was, as you point out, available with a 2.8L for a while so the 60-degree stuff also all works. In the Midwest, you can still find 3.1s all day in junkyards from Monte Carlos and Luminas and Centurys. That's probably a better middle ground if you aren't a carb guy and you can upgrade to the 3.4, 3.5, or 3.9 even if you feel so compelled over time.
The 3800 has a bunch of the upsides of the 2.8/3.1/etc. and is probably a better road-racing engine, in theory. The torque curve is basically flat so there's no need to rev the hell out of them and they're pretty robust. But they do make quite a bit more power and torque than the other engine families so you'll probably need to do a bunch of other upgrades like brakes and you'll have to be gentle with the transmission (Paging OnkelUdo to the Busted Cheaty T5 Phone).
OR if you wanted to chase another IOE, you could:
* Cut your power in half (51 crank horsepower) and run an Isuzu Diesel from a Chevette or an I-Mark. I even know where you could find a couple of them just 2-3 hours away. You'd be able to race the Simca 1204 straight-up (ish) and get 30 mpg on the track.
* Find a Chevy "Super-Thrift" 153 somewhere. AMC used them in the DJ-5 and they were a base-base-base option on Chevy IIs in the '60s. Nobody in the world wants them, but Mercruiser made (and makes, present tense) a marine version you could probably find cheaply. Plenty of people have run Mercruiser 4.3s, by the way, in Lemons. The shitty old gray Monza that resurfaces periodically has one.
Edit to clarify: Don't buy a new Mercruiser engine, that's stupid. But plenty of people have boats they don't want or that they don't use. The 4.3s we've seen, one came out of a moldy house boat and another was stolen from a neighbor's unused boat or something.
* GMC V6. Talk about heavy V6s, these things are massive and we've been dying for someone to put one in a Lemons car for ages. There's also a V12 version that was special order in Canada with 702 cubic inches for 250 HP/585 lb.-ft. torque.
* Or go find an Opel V6 from a Cadillac if you really hate yourself. Overhead Cams!
Everything Bagel, 24 Hours of Lemons