Man, I really feel for ya. Been there, too. My 1st 440, hyd roller, ported eddy heads, all 1st class parts & custom paint. Loaded it up from the engine builder's shop & took it to Mallory's dyno & produced some good numbers. Was really stoked until a friend pointed out the bolt hole in the
curved bell housing mount on the block was sheared off at the hole.
Builder says block was that way when we brought it in...
Then my bro spent a ton on a roller cammed 472 Hemi that was ready to dyno & he had a gut feeling it wasn't up to snuff after a valve tip sheared off on his 440 built by the same guy that did his Hemi. Shipped the Hemi to Arruzza & John documented enough stuff that said it wouldn't have lasted 2 minutes on the dyno. No cam button & tons of other bonehead mistakes.
John built him a killer, reliable 518 that'll fry the tires at 50.
Muscle Motors should never have taken your money if they knew they couldn't deliver or never did one before with those heads. I can imagine the bill was huge & now you've got to eat it or trust these boneheads to make it right. I'd ship it to Best & have them assess the motor & see what they can do to. Best wishes in whatever happens.