From what I understand after reading post after post is the on the distributor cap you may or may not have a 8 and 6. The 8 is for V8 engines and the 6 is for the V6 engine. There also may be two timming marks on the harmonic balance on the crankshaft.

There is on mine which I didn't realize until read every post i could find on every forum i could find.

Since you have the 4.3 v6, and more than likely have two timming marks. I believe that you need to pull the distributor and roll the engine over by hand clockwise until the two marks on the balancer line up, mine were at about one and five o'clock, this should be TDC on the number one cylinder either on the compression stroke or exhaust stroke. I pulled the spark plug on the number one cylinder and listen for air to rush out. Pop the distributor back in and the rotor should point to the 6 on the cap, if it doesn't it may because of the oil pump drive isn't lined up right. Which is a PIA to line up, at least on my blazer it was you need a long standard screwdriver and a mirror, I used the camera on my cell phone. Put the cap back on and try to fire it up, I got lucky and it fired right up, if it doesn't you probably didn't get it lined up on compression stroke and you lined it up on the exhaust. if this happens roll the engine over until the rotor is opposite on the 6, pull the distributor again and put it back in and line it up with the 6 again and put it together. it should fire up.

I don't know for sure if it will work but it made sense to me when i read it and tried it and it worked.

 

I know this is an older post and you may have resolved the issue. And I may just be another idiot on the web but....

I did gaskets on my 99 Tahoe and had a similar problem. I had spark and compression. There was fuel all the way up to the injectors... but it just wouldn't start. As it turns out it was an issue with the factory on board anti theft system. What it does is shuts down the signal to the injectors when it has been tampered with or is faulty some how. So it sounds like it wants to start or actually starts but does not run at all. (dies out immediately!!) Unfortunately I do not know much more about it and how to fix it if it has been triggered somehow. In my case the fuse was blown. I replaced it and it started right up.

It may sound ridiculous and it did to me too but sometimes you actually have to grasp that straw.

I wish I could say the same for my 2003 S10 that now seems to be acting the same way after new gaskets. You would think I would learn...

 

im having the same problem with my blazer as well. i believe the distributer is off. its a pain to get in the right spot.