This has all the red flags of a rip off builder. *$850 to assemble a SBC that you are supplying quite a bit of the parts for isn't all that cheap. Did you even ask about 2 vs. 4 bolt main? Mfg. of the other parts? It all makes a big difference believe me.
In the same complex as your guy is an OG builder named Karl. He has been in business in Corona for nearly 40 years and in the current shop for 30. He is in the same business park as your guy is. Give him a call and see what Karl says about building you a stout motor.
http://www.karlsautomachineshop.com/I took my Toyota 4x4 22R to be rebuilt by a place that advertised 100,000 mile warranty for a $1400 rebuild. Turns out they build 'em like they do in the outback of Mexico. Never take the block out of the truck, hone everything in the truck and put it back together. When it wouldn't even drive home across town and he wouldn't honor squat, I went to small claims court with him. He already had 30 cases pending, never showed up, and apparently nothing is in his name so I couldn't go after the business or him personally. But he did run a half page ad in the Auto Trader so he must be good right?