This question has several answers, depending upon what you want, and how you want to get your engine installed.
If you are in Northern California, near the SF Bay area, also consider Craig Hill at Top of the Hill Performance.
Here's the website:
http://www.topofthehillpc.com/midframe.html
Craig did my engine; I raced and drove it for two years afterward. It delivered 445 HP and 437 lb.ft. at the flywheel from a carbureted engine running on premium pump gas! All this from a 351W small block.
Good luck with whoever you choose, as there are several good engine builders.
All the best,