I paid approx 18 grand for a carbed 540 , my block … Dart cnc heads etc .. dyno tuned and tested .. a little north of 700 hp @5600 .. turning accys with dyno headers.. carb,intake , heads , etc all new. New oil pan and larger oil cooler , ignition , starter , all new. I did re use my closed cooling system and accys as I wanted to keep a serpentine setup.

I do think the main issue (OP correct me if I'm wrong) is the 30% ($10K) jump on agreed cost
That would get me no matter who does the build

What paperwork does the OP have to support the agreed $27K?..If it's in writing different story than no paperwork... also different outcome if there was a lot of add on work/accessories after the agreed to quote.

I've been bit by that one my self. Hand shakes are not what they use to be