Some people may advise you against this advice I am about to give, and if someone has more recent experience, then you should not listen to me...
I bought a motor from City Motor Supply in Dallas 9 years ago and it has always been very strong and run well. I was going to do the same thing again, but some people advised me against it. I don't know how they are now, but I can tell you for sure that they used to build a great motor, stock-wise. And for a short block, you would only pay $350 for a 351m.
Either way, I think that CMS would be better than an Auto Zone Engine.
thats what I am doing....going with a 400 crank and 400 flattops. The rest is stock in the bottom. and yes, you are right, the machine shop building it wasn't much more than buying a stock one. prices are much better than they were a few years ago.