I have no experience with DSS, but I can vouch for Keith Craft Racing.
I purchased a "built" 306-ci short block from KCR in Sept 2006 for a twin-turbocharged application. Over 50K miles of 9# boost through that motor with no issues. Great service, fair price, and no complaints. I would use them again.
My only word of wisdom is to unpack the motor and inspect it for "shipping damage" before the delivery guy leaves. The monkey that delivered mine to my door step, roughly dropped the pallet to the ground in my driveway and cracked the front timing cover. I regret not taking the 2-3 minutes to thoroughly look it over before the delivery guy left. It was a easy fix of course, but cost me time and a new timing cover...not KCR's fault either.
HTH,
Harold