Okay, skip the 5.3 dude, the 4.3 is just a shortened 5.7. If you replace it with the 5.7 it will be a direct fit to your tranny, and very slight engine mount mods. Make sure though that if you go with a newer motor that you replace the brain, the comp needs to be able to fire 8 cylinders instead of 6.
I have a 1996 s10 and I have actually done extensive research on a v8 swap. I am telling you dude, you will be much happier with the 350.