Engine shop woes

You got a 3yr warrenty, put it in and fire it up.
$2600 is cheaper than Jasper rebuilds that NAPA sells.
As far as the balance issue, they may of have to drill holes in the crank instead of adding weight.
I really think you are worring over nothing, but I would still want the specs on the motor.
I bet that motor will fire right up and run with no issues.