You can pick up a 5.3 liter out of a truck complete for 500 bucks. Thats with an ECU. Get some 4.8 pistons out of a junk yard and toss them in to bump the compression. Buy the mounts, throw it in, have a mid pipe made for the exhaust, and your sittin at ~280 horse for about a grand. From there, typical bolt on shit and you can make 400 horse N/A with the stock bore and stroke 5.3 no problem. Or just throw a turbo on it and boost it to ~400 horse. The 4.8/5.3 have the best blocks for boost cuz theyre iron, and they have super thick cylinder walls. The stock crank is good for about 800 horse, rods and piston start to get iffy at about 400. Its the cheap way to have a fast S10.
If you REALLY want to keep the 4.3, spray and pray. They do well with nitrous, so spray the shit out of it.