I have had to have the original engine in my matching numbers '72 re-built, other than a mild cam it will be pretty much to stock...it was one tired old 350. I'm hearing all kinds of stories about things that go wrong/don't work after an engine has been reinstalled so I would like some advice from the members on your experiences...please.

I can only attest to what went wrong with my reinstall.  A real schmutz did it for me.  The one thing I can say is that you should look in the engine compartment now that the engine is out and check for bad vacuum hoses and any wires that may need to be replaced.  Look at all the components that are easier to inspect now that you have some walking around room under the hood.

It's probably too late for you now; but I like to take a lot of pictures before I tear something apart especially under the hood. Manuals are good but there is nothing like a photo of the real deal. 

#1: Use the correct engine break-in oil! Preferably one with a high ZDDP content, or use a Zinc additive for engine break-in. Do NOT let the engine idle on start-up. It needs to run at 2000/2500 rpms on initial start-up for around 20 minutes. If you allow it to idle slow, it will wipe out the cam lobes and bottoms of the lifters. This is the number 1 problem most folks have with a fresh engine start-up. So...make sure everything is right...timing, fuel, coolant, other fluids, etc., so it will fire up and run correctly without having to shut it down, or dying on it's own.
Check for ANY leaks immediately on start-up.
Have a good fan to put in front of the radiator to keep plenty of air blowing to help keep it from overheating.
If you have new headers with a coating on them...you're not gunna like what will happen to them if you use them during the engine break-in period, so use something else instead of new headers, if possible.
BE sure to have the trans in Park/Neutral, wheels chocked, or off the ground, in case it fires up in gear(don't ask...)

I agree with Joel - break-in procedure is key.  The only work I shopped out on mine was the machine work and putting the heads back together, so the "schmutz" in my case was me!  I experienced a few pitfalls  with the break-in: 1) primary side of distributor was not connected securely (I think) so I was getting no spark and I cranked on it way too much before I realized it.  2) the return line at the fuel pump was not tight enough so I had a minor fuel leak in the middle of break in and had to shut it down and restart.  Fortunately, cam appears to be OK so far after over 1K miles, but but it was nerve wracking and it still could have caused damage.  I used Comp Cams break-in oil and now am using regular conventional motor oil (10W-30) with a ZDDP additive.  

Guys,

See if the builders offer to break in the engine.  There are quite a few who do. That is something I would have done had I known I wouldn't have been able to install the engine myself.