There are number of reasons that there are much better choices than an old TBI 5.7 for your 99. The fact it is only 5HP more than a 4.3 was in 99 is one of them. To make the TBI work there would be a lot of wiring changes needed.
A 96-02 L31 would be a better choice at 255HP, a 65HP increase over the 4.3 and double your Tutu. Installation would be much easier. You would simply need a 4.3 S10 PCM and wiring harness and only need to add 4 wires. Since the 2.2 trans won't work with either getting a 2wd 5.7 4L60E trans along with the engine would be best. The L31's with 2wd are fairly common in 96-02 vans.
You can probably sell your TBI for about the same amount the L31 will cost since good 350 TBI's are getting somewhat scarce. Your rear end will be fine with either engine and has a good performance ratio. Won't get killer gas mileage, tho. Doubt there's much chance it's LSD. The Silverado rear won't fit in an S10.
About all you'll need besides the 4.3 engine harness and PCM are mounts, headers, and moving the radiator forward a bit since the V8 is longer. You'll need a 4.3 rad and support for either engine.
As
@Rhotpursuit said read the complete V8 swap section. It's got a lot of info and will take some time, but you'll learn a lot and in the long run save both time and money.