How far can you get?  Without more descriptions of the problem, it is hard to know what to suggest.  

Can you get the block fully into the breech?

Can you do this with the extractor in place too?  
If yes to the first and no to the second, then the issue may be that you have not found the sweet spot to hold the extractor while inserting the block.  Keep fiddlin' with the block about 2/3s of the way in.  Watching from the top of the receiver as you do this may help.

Can you get the pin almost all the way home or only a very short fraction of the total distance?  

You should insert the pin when the lever more or less vertical.  If you can get the pin in only a short way, I am betting you are holding the lever in the up position.

If you can get it most of the way in, then I would wager you have the pin at the wrong angle.  The pin should be rotated so that the lever is pointing close to straight forward.  This will allow the flange at the base of the pin to clear the bump-out on the receiver.  When the pin is fully inserted, the lever is rotated about 90 degrees to straight up and this will lock it in place with the flange sliding into the groove in the receiver.

Those are my best guesses until you you tell us more.  

Brent

Let me add, if you can't get the block in with these instructions, then one more thing - can you get the pin in w/o the block or extractor?

dd, I modifidied my post above.  Check that you have the pin's lever pointing forward or even a little downward from straight forward.  Watch where the eccentric lip on the lever fits into the groove in the receiver's bump out.   

Brent

How about relaxing the lever tension spring?
Good luck, Joe

I loosned the lever spring and I was able to reassemble the rifle. I dont think you should have to do that every time to clean or lube. Is there a specific tension that spring should be under. Maybe it was too tight or too loose. Thoughts?

Mr. Dirtdog,

I have a very nice old highwall that someone took apart but couldn't get back together, so they started wanging on the bottom of the breecblock with a hammer. What our hero (I think I know who he is) couldn't figure out was that Winchesters have a firing pin retractor and so you have to shift the lever forward a little so the the hammer falls against the block, and then the breech slides in hammer and all. I don't know if your rifle has a retractor or not, but please don't get mad and beat on your gun, there must be some way to get it together.

Bob

It was my post.

Later Sharps designs like the Borchardt had special long-headed lever spring screws with slotted heads that projected up level with the surface of the wood, for external loosening & retightening during field-stripping. A very strong and determined person might be able to assemble a sidehammer with the spring tight, but.....of course you might get away with tapering the nose of the pin enough to be able to brute-force it into place.

Please be advised that this spring is, historically, one of the two weakest points of your action. Sidehammer Sharps of most persuasions have a rep for breaking this spring and the firing pin, so please don't be surprised if it happens. A simple approach might be to provide a leather or rubber shim under the spring, along with possibly a slightly-relaxed screw tension, to reduce the tendency toward breakage.
Good luck, Joe

I went through the same anguish when I first took down and then reassembled my Shiloh '74.  I could not understand why the designers back in the 1800s would put that nice little "button" on the side of the action to rotate and remove the lever pin and then make it so diificult to get back together.  I decided it was time well spent removing the forearm and lever spring anytme I had to remove and reinstal the breach block.  It almost jumps in by itself with that spring out of the way   THOP