383 should fit just fine, still just a 350 block. I have seen small block chevys swapped into '95 up blazers, and they almost seem to have more room under the hood then the earlier models like my old '91 4x4 *-10 blazer that I swapped in a small block.
If you go with a small block you will need new motor mounts, I would suggest a bigger radiator, and definatly an aftermarket trans cooler since there would be alot more heat under the hood.
if the blazer is a 4x4, you will also need a custom oil pan to fit around the front axle housing, I looked around and there are plenty of options from buying the whole pan to buying a weld in piece to make the stock oil pan from the 4.3 work on the 5.7.
you will also need an oil filter relocation kit if the stock adapter from the v6 won't fit onto the v8
there are also kits to make the stock fuel injection work with the small block.
you will definatly have some money wraped up in a project like this, but if you know how to turn some wrenches and do a little custom fabwork, you can save some money.
some kit pieces can be found at http://www.advanceadapters.com/
it may take some time and money, but the outcome is great, these things are crazy fast with a healhy v8, even a bone stock 350
If you decide to go this route and have a few questions, look me up, I will be glad to answer any questions that you might have if I know the answer, of course